[llvm] [CodeGen][NFC] Guard copy propogation in machine CSE against undefs (PR #97413)

[Matt Arsenault via llvm-commits]

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@@ -184,7 +184,7 @@ bool MachineCSE::PerformTrivialCopyPropagation(MachineInstr *MI,
       continue;
     bool OnlyOneUse = MRI->hasOneNonDBGUse(Reg);
     MachineInstr *DefMI = MRI->getVRegDef(Reg);
-    if (!DefMI->isCopy())
+    if (!DefMI || !DefMI->isCopy())
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arsenm wrote:

I think it would be better if we moved towards making undef operands a verifier error for SSA MIR. These undefined reg checks really should be unnecessary 

https://github.com/llvm/llvm-project/pull/97413