[GlobalISel] fix crash in narrowScalarExtract if DstRegs only has one register

Summary: When narrowing a scalar G_EXTRACT where the destination lines up perfectly with a single result of the emitted G_UNMERGE_VALUES a COPY should be emitted instead of unconditionally trying to emit a G_MERGE_VALUES.

Reviewers: arsenm, dsanders

Reviewed By: arsenm

Subscribers: wdng, rovka, hiraditya, volkan, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D75743
diff --git a/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp b/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
index 6384cc9..d0a51ed 100644
--- a/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
@@ -3703,10 +3703,12 @@
   }
 
   Register DstReg = MI.getOperand(0).getReg();
-  if(MRI.getType(DstReg).isVector())
+  if (MRI.getType(DstReg).isVector())
     MIRBuilder.buildBuildVector(DstReg, DstRegs);
-  else
+  else if (DstRegs.size() > 1)
     MIRBuilder.buildMerge(DstReg, DstRegs);
+  else
+    MIRBuilder.buildCopy(DstReg, DstRegs[0]);
   MI.eraseFromParent();
   return Legalized;
 }