[llvm] [AArch64][GlobalISel] Constrain G_CONSTANT_FOLD_BARRIER operand register classes (PR #177997)

[Matt Arsenault via llvm-commits]

================
@@ -366,10 +366,11 @@ bool InstructionSelect::selectInstr(MachineInstr &MI) {
     //
     // Propagate that through to the source register.
     const TargetRegisterClass *DstRC = MRI.getRegClassOrNull(DstReg);
-    if (DstRC)
+    const TargetRegisterClass *SrcRC = MRI.getRegClassOrNull(SrcReg);
+    if (DstRC && SrcRC)
+      MRI.constrainRegClass(SrcReg, DstRC);
+    else if (DstRC)
       MRI.setRegClass(SrcReg, DstRC);
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arsenm wrote:

Probably doesn't work if the class wasn't set in the first place, maybe
`if (!SrcRC)
    MRI.setRegClass(SrcReg, DstRC)
else
  MRI.constrainRegClass
`
?

https://github.com/llvm/llvm-project/pull/177997