AMDGPU: Fix a corner case crash in SIOptimizeExecMasking

[nhaehnle]

Summary

See the new test case; this is really unlikely to happen with real code,
but I ran into this while attempting to bugpoint-reduce a different issue.

Change-Id: I9ade1dc1aa8fd9c4d9fc83661d7b80e310b5c4a6

[rampitec]

LGTM

llvm/trunk/lib/Target/AMDGPU/SIOptimizeExecMasking.cpp

 }
 
 /// If \p MI is a copy to exec, return the register copied from.
 static unsigned isCopyToExec(const MachineInstr &MI) {
   switch (MI.getOpcode()) {
   case AMDGPU::COPY:
   case AMDGPU::S_MOV_B64: {
     const MachineOperand &Dst = MI.getOperand(0);
-    if (Dst.isReg() && Dst.getReg() == AMDGPU::EXEC)
+    if (Dst.isReg() && Dst.getReg() == AMDGPU::EXEC && MI.getOperand(1).isReg())
       return MI.getOperand(1).getReg();
     break;
   }
   case AMDGPU::S_MOV_B64_term:
     llvm_unreachable("should have been replaced");
   }
 
   return AMDGPU::NoRegister;

llvm/trunk/test/CodeGen/AMDGPU/llvm.amdgcn.init.exec.ll

 define amdgpu_ps float @reuse_input2(i32 inreg %count, i32 %a) {
 main_body:
   %s = add i32 %a, %count
   %f = sitofp i32 %s to float
   call void @llvm.amdgcn.init.exec.from.input(i32 %count, i32 19)
   ret float %f
 }
 
+; GCN-LABEL: {{^}}init_unreachable:
+;
+; This used to crash.
+define amdgpu_ps void @init_unreachable() {
+main_body:
+  call void @llvm.amdgcn.init.exec(i64 -1)
+  unreachable
+}
+
 declare void @llvm.amdgcn.init.exec(i64) #1
 declare void @llvm.amdgcn.init.exec.from.input(i32, i32) #1
 
 attributes #1 = { convergent }