[GlobalISel] Fix G_SEXT narrowScalar to bail out of unsupported type combination.

Similar to the issue with G_ZEXT that was fixed earlier, this is a quick to fall back if the source type is not exactly half of the dest type. Fixes the clang-cmake-aarch64-lld bot build. llvm-svn: 370847

Author: aemerson

llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp

@@ -619,13 +619,17 @@ LegalizerHelper::LegalizeResult LegalizerHelper::narrowScalar(MachineInstr &MI,
  if (TypeIdx != 0)
  return UnableToLegalize;
 
- if (NarrowTy.getSizeInBits() != SizeOp0 / 2) {
+ Register SrcReg = MI.getOperand(1).getReg();
+ LLT SrcTy = MRI.getType(SrcReg);
+
+ // FIXME: support the general case where the requested NarrowTy may not be
+ // the same as the source type. E.g. s128 = sext(s32)
+ if ((SrcTy.getSizeInBits() != SizeOp0 / 2) ||
+ SrcTy.getSizeInBits() != NarrowTy.getSizeInBits()) {
  LLVM_DEBUG(dbgs() << "Can't narrow sext to type " << NarrowTy << "\n");
  return UnableToLegalize;
  }
 
- Register SrcReg = MI.getOperand(1).getReg();
-
  // Shift the sign bit of the low register through the high register.
  auto ShiftAmt =
  MIRBuilder.buildConstant(LLT::scalar(64), NarrowTy.getSizeInBits() - 1);