Add a nop instruction if a section starts with landing pad for function splitter

This change adds a nop instruction if section starts with landing pad. This change is like [D73739](https://reviews.llvm.org/D73739) which avoids zero offset landing pad in basic block sections.

Detailed description:
The current machine functions splitter can create ˜sections which start with a landing pad themselves. This places landing pad at offset zero from LPStart.
```
	.section	.text.split.foo10,"ax",@progbits
foo10.cold:                             # %lpad
	.cfi_startproc
	.cfi_personality 3, __gxx_personality_v0
	.cfi_lsda 3, .Lexception5
	.cfi_def_cfa %rsp, 16
.Ltmp11: <--- This is a Landing pad and also LP Start as it is start of this section
	movq	%rax, %rdi <--- first instruction is at offest 0 from LPStart
	callq	_Unwind_Resume@PLT

 ```
This will cause landing pad entries to become zero (.Ltmp11-foo10.cold)
```
.Lcst_begin4:
	.uleb128 .Ltmp9-.Lfunc_begin2           # >> Call Site 1 <<
	.uleb128 .Ltmp10-.Ltmp9                 #   Call between .Ltmp9 and .Ltmp10
	.uleb128 .Ltmp11-foo10.cold  <---This is zero           #     jumps to .Ltmp11
	.byte	3                               #   On action: 2
	.uleb128 .Ltmp10-.Lfunc_begin2          # >> Call Site 2 <<
	.uleb128 .Lfunc_end9-.Ltmp10            #   Call between .Ltmp10 and .Lfunc_end9
	.byte	0                               #     has no landing pad
	.byte	0                               #   On action: cleanup
	.p2align	2
```
The C++ ABI somehow assumes that no landing pads point directly to LPStart (which works in the normal case since the function begin is never a landing pad), and uses LP.offset = 0 to specify no landing pad. This change adds a nop instruction at start of such sections so that such a case could be avoided. Output:
```
	.section	.text.split.foo10,"ax",@progbits
foo10.cold:                             # %lpad
	.cfi_startproc
	.cfi_personality 3, __gxx_personality_v0
	.cfi_lsda 3, .Lexception5
	.cfi_def_cfa %rsp, 16
	nop <--- new instruction that is added
.Ltmp11:
	movq	%rax, %rdi
	callq	_Unwind_Resume@PLT
```

Reviewed By: modimo, snehasish, rahmanl

Differential Revision: https://reviews.llvm.org/D130133

llvm/include/llvm/CodeGen/BasicBlockSectionUtils.h

@@ -25,6 +25,8 @@ using MachineBasicBlockComparator =
 void sortBasicBlocksAndUpdateBranches(MachineFunction &MF,
                                       MachineBasicBlockComparator MBBCmp);
 
+void avoidZeroOffsetLandingPad(MachineFunction &MF);
+
 } // end namespace llvm
 
 #endif // LLVM_CODEGEN_BASICBLOCKSECTIONUTILS_H

llvm/lib/CodeGen/BasicBlockSections.cpp

@@ -268,8 +268,8 @@ void llvm::sortBasicBlocksAndUpdateBranches(
 // If the exception section begins with a landing pad, that landing pad will
 // assume a zero offset (relative to @LPStart) in the LSDA. However, a value of
 // zero implies "no landing pad." This function inserts a NOP just before the EH
-// pad label to ensure a nonzero offset. Returns true if padding is not needed.
-static bool avoidZeroOffsetLandingPad(MachineFunction &MF) {
+// pad label to ensure a nonzero offset.
+void llvm::avoidZeroOffsetLandingPad(MachineFunction &MF) {
   for (auto &MBB : MF) {
     if (MBB.isBeginSection() && MBB.isEHPad()) {
       MachineBasicBlock::iterator MI = MBB.begin();
@@ -278,10 +278,8 @@ static bool avoidZeroOffsetLandingPad(MachineFunction &MF) {
       MCInst Nop = MF.getSubtarget().getInstrInfo()->getNop();
       BuildMI(MBB, MI, DebugLoc(),
               MF.getSubtarget().getInstrInfo()->get(Nop.getOpcode()));
-      return false;
     }
   }
-  return true;
 }
 
 // This checks if the source of this function has drifted since this binary was

llvm/lib/CodeGen/MachineFunctionSplitter.cpp

@@ -146,7 +146,7 @@ bool MachineFunctionSplitter::runOnMachineFunction(MachineFunction &MF) {
     return X.getSectionID().Type < Y.getSectionID().Type;
   };
   llvm::sortBasicBlocksAndUpdateBranches(MF, Comparator);
-
+  llvm::avoidZeroOffsetLandingPad(MF);
   return true;
 }
 

llvm/test/CodeGen/X86/machine-function-splitter.ll

@@ -242,6 +242,28 @@ define void @foo9(i1 zeroext %0) nounwind #0 !prof !14 {
   ret void
 }
 
+define i32 @foo10(i1 zeroext %0) personality ptr @__gxx_personality_v0 !prof !14 {
+;; Check that nop is inserted just before the EH pad if it's beginning a section.
+; MFS-DEFAULTS-LABEL: foo10
+; MFS-DEFAULTS-LABEL: callq   baz
+; MFS-DEFAULTS:       .section        .text.split.foo10,"ax",@progbits
+; MFS-DEFAULTS-NEXT:  foo10.cold:
+; MFS-DEFAULTS:       nop
+; MFS-DEFAULTS:       callq   _Unwind_Resume@PLT
+entry:
+  invoke void @_Z1fv()
+          to label %try.cont unwind label %lpad, !prof !17
+
+lpad:
+  %1 = landingpad { ptr, i32 }
+          cleanup
+          catch ptr @_ZTIi
+  resume { ptr, i32 } %1
+
+try.cont:
+  %2 = call i32 @baz()
+  ret i32 %2
+}
 
 declare i32 @bar()
 declare i32 @baz()