[ARM] Remove FeatureNoARM implies ModeThumb.

Summary:
By removing FeatureNoARM implies ModeThumb, we can detect cases where a
function's target-features contain -thumb-mode (enables ARM codegen for the
function), but the architecture does not support ARM mode. Previously, the
implication caused the FeatureNoARM bit to be cleared for functions with
-thumb-mode, making the assertion in ARMSubtarget::ARMSubtarget [1]
pointless for such functions.

This assertion is the only guard against generating ARM code for
architectures without ARM codegen support. Is there a place where we
could easily generate error messages for the user? At the moment, we
would generate ARM code for Thumb-only architectures. X86 has the same
behavior as ARM, as in it only has an assertion and no error message,
but I think for ARM an error message would be helpful. What do you
think?

For the example below, `llc -mtriple=armv7m-eabi test.ll -o -` will
generate ARM assembler (or fail with an assertion error with this patch).
Note that if we run the resulting assembler through llvm-mc, we get
an appropriate error message, but not when codegen is handled
through clang.

```
define void @bar() #0 {
entry:
  ret void
}

attributes #0 = { "target-features"="-thumb-mode" }
```

[1] c1f7b54cef/lib/Target/ARM/ARMSubtarget.cpp (L147)

Reviewers: t.p.northover, rengolin, peter.smith, aadg, silviu.baranga, richard.barton.arm, echristo

Reviewed By: rengolin, echristo

Subscribers: efriedma, aemerson, javed.absar, kristof.beyls, llvm-commits

Differential Revision: https://reviews.llvm.org/D35569

llvm-svn: 310476

llvm/lib/Target/ARM/ARM.td

@@ -341,9 +341,7 @@ def FeatureThumb2 : SubtargetFeature<"thumb2", "HasThumb2", "true",
                                      "Enable Thumb2 instructions">;
 
 def FeatureNoARM  : SubtargetFeature<"noarm", "NoARM", "true",
-                                     "Does not support ARM mode execution",
+                                     "Does not support ARM mode execution">;
-                                     [ModeThumb]>;
-
 
 //===----------------------------------------------------------------------===//
 // ARM ISAa.
@@ -504,11 +502,13 @@ def ARMv6kz   : Architecture<"armv6kz",   "ARMv6kz",  [HasV6KOps,
 
 def ARMv6m    : Architecture<"armv6-m",   "ARMv6m",   [HasV6MOps,
                                                        FeatureNoARM,
+                                                       ModeThumb,
                                                        FeatureDB,
                                                        FeatureMClass]>;
 
 def ARMv6sm   : Architecture<"armv6s-m",  "ARMv6sm",  [HasV6MOps,
                                                        FeatureNoARM,
+                                                       ModeThumb,
                                                        FeatureDB,
                                                        FeatureMClass]>;
 
@@ -536,6 +536,7 @@ def ARMv7r    : Architecture<"armv7-r",   "ARMv7r",   [HasV7Ops,
 def ARMv7m    : Architecture<"armv7-m",   "ARMv7m",   [HasV7Ops,
                                                        FeatureThumb2,
                                                        FeatureNoARM,
+                                                       ModeThumb,
                                                        FeatureDB,
                                                        FeatureHWDivThumb,
                                                        FeatureMClass]>;
@@ -543,6 +544,7 @@ def ARMv7m    : Architecture<"armv7-m",   "ARMv7m",   [HasV7Ops,
 def ARMv7em   : Architecture<"armv7e-m",  "ARMv7em",  [HasV7Ops,
                                                        FeatureThumb2,
                                                        FeatureNoARM,
+                                                       ModeThumb,
                                                        FeatureDB,
                                                        FeatureHWDivThumb,
                                                        FeatureMClass,
@@ -598,6 +600,7 @@ def ARMv8r    : Architecture<"armv8-r",   "ARMv8r",   [HasV8Ops,
 def ARMv8mBaseline : Architecture<"armv8-m.base", "ARMv8mBaseline",
                                                       [HasV8MBaselineOps,
                                                        FeatureNoARM,
+                                                       ModeThumb,
                                                        FeatureDB,
                                                        FeatureHWDivThumb,
                                                        FeatureV7Clrex,
@@ -608,6 +611,7 @@ def ARMv8mBaseline : Architecture<"armv8-m.base", "ARMv8mBaseline",
 def ARMv8mMainline : Architecture<"armv8-m.main", "ARMv8mMainline",
                                                       [HasV8MMainlineOps,
                                                        FeatureNoARM,
+                                                       ModeThumb,
                                                        FeatureDB,
                                                        FeatureHWDivThumb,
                                                        Feature8MSecExt,

llvm/test/CodeGen/ARM/scavenging.mir

@@ -1,4 +1,4 @@
-# RUN: llc -o - %s -mtriple=arm-arm-none-eabi -mcpu=cortex-m0 -run-pass scavenger-test | FileCheck %s
+# RUN: llc -o - %s -mtriple=thumb-arm-none-eabi -mcpu=cortex-m0 -run-pass scavenger-test | FileCheck %s
 ---
 # CHECK-LABEL: name: scavengebug0
 # Make sure we are not spilling/using a physreg used in the very last