[CodeGen] Fix a trivial type conversion bug dating back to pre-2008

The heuristic above this code is incredibly suspect, but disregarding that it mutates the cast opcode so we need to check the *mutated* opcode later to see if we need to emit an AssertSext or AssertZext node.

Fixes PR29041.

llvm-svn: 279223

llvm/lib/CodeGen/SelectionDAG/LegalizeIntegerTypes.cpp

@@ -427,7 +427,7 @@ SDValue DAGTypeLegalizer::PromoteIntRes_FP_TO_XINT(SDNode *N) {
   // Assert that the converted value fits in the original type.  If it doesn't
   // (eg: because the value being converted is too big), then the result of the
   // original operation was undefined anyway, so the assert is still correct.
-  return DAG.getNode(N->getOpcode() == ISD::FP_TO_UINT ?
+  return DAG.getNode(NewOpc == ISD::FP_TO_UINT ?
                      ISD::AssertZext : ISD::AssertSext, dl, NVT, Res,
                      DAG.getValueType(N->getValueType(0).getScalarType()));
 }

llvm/test/CodeGen/AArch64/fptouint-i8-zext.ll

@@ -0,0 +1,15 @@
+; RUN: llc < %s | FileCheck %s
+
+target datalayout = "e-m:e-i8:8:32-i16:16:32-i64:64-i128:128-n32:64-S128"
+target triple = "aarch64"
+
+; CHECK-LABEL: float_char_int_func:
+; CHECK: fcvtzs [[A:w[0-9]+]], s0
+; CHECK-NEXT: and w0, [[A]], #0xff
+; CHECK-NEXT: ret
+define i32 @float_char_int_func(float %infloatVal) {
+entry:
+  %conv = fptoui float %infloatVal to i8
+  %conv1 = zext i8 %conv to i32
+  ret i32 %conv1
+}