[GlobalISel] Fix a crash when handling an invalid MVT during call lowering.

This crash was introduced in r358032 as we try to construct an EVT from an MVT
in order to find the register type for the calling conv. Fall back instead of
trying to do this with an invalid MVT coming from i256.

llvm-svn: 358314

llvm/lib/CodeGen/GlobalISel/CallLowering.cpp

@@ -125,7 +125,7 @@ bool CallLowering::handleAssignments(MachineIRBuilder &MIRBuilder,
     MVT CurVT = MVT::getVT(Args[i].Ty);
     if (Handler.assignArg(i, CurVT, CurVT, CCValAssign::Full, Args[i], CCInfo)) {
       // Try to use the register type if we couldn't assign the VT.
-      if (!Handler.isArgumentHandler())
+      if (!Handler.isArgumentHandler() || !CurVT.isValid())
         return false; 
       CurVT = TLI->getRegisterTypeForCallingConv(
           F.getContext(), F.getCallingConv(), EVT(CurVT));

llvm/test/CodeGen/AArch64/GlobalISel/call-lowering-i256-crash.ll

@@ -0,0 +1,7 @@
+; RUN: llc -mtriple=aarch64-linux-gnu -O0 -verify-machineinstrs -o - %s | FileCheck %s
+
+define i1 @test_crash_i256(i256 %int) {
+; CHECK-LABEL: test_crash_i256
+; CHECK: ret
+  ret i1 true
+}