Cancelling out G_MERGE_VALUES/G_UNMERGE pairs sometimes needs a copy

[dsanders]

Summary

Specifically, if there are register class constraints they must be
preserved. If there is at least one, then we must emit a copy unless both
vregs have the same constraint.

[arsenm]

Needs tests

llvm/include/llvm/CodeGen/GlobalISel/LegalizationArtifactCombiner.h

402

This is just OldRC == NewRC

406

buildCopy

[dsanders]

+Testcase

[dsanders]

Fix the remaining small issues

[arsenm]

LGTM

llvm/include/llvm/CodeGen/GlobalISel/LegalizationArtifactCombiner.h

if (NumMergeRegs < NumDefs) {
       }
 
       assert(DestTy == MergeSrcTy &&
              "Bitcast and the other kinds of conversions should "
              "have happened earlier");
 
       for (unsigned Idx = 0; Idx < NumDefs; ++Idx) {
         Register NewDef = MergeI->getOperand(Idx + 1).getReg();
-        MRI.replaceRegWith(MI.getOperand(Idx).getReg(), NewDef);
+        Register OldReg = MI.getOperand(Idx).getReg();
+        auto *OldRC = MRI.getRegClassOrNull(OldReg);
+        auto *NewRC = MRI.getRegClassOrNull(NewDef);
+        if ((!OldRC && !NewRC) || (OldRC && NewRC && OldRC == NewRC))
+          MRI.replaceRegWith(OldReg, NewDef);
+        else {
+          Builder.setInstr(MI);
+          Builder.buildInstr(TargetOpcode::COPY, {OldReg}, {NewDef});
+        }
         UpdatedDefs.push_back(NewDef);
       }
     }
 
     markInstAndDefDead(MI, *MergeI, DeadInsts);
     return true;
   }