[llvm] CodeGen: Fix lookThruCopyLike crash on an undef register (PR #216598)

[Matt Arsenault via llvm-commits]

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@@ -608,7 +608,7 @@ TargetRegisterInfo::lookThruCopyLike(Register SrcReg,
                                      const MachineRegisterInfo *MRI) const {
   while (true) {
     const MachineInstr *MI = MRI->getVRegDef(SrcReg);
-    if (!MI->isCopyLike())
+    if (!MI || !MI->isCopyLike())
       return SrcReg;
 
     Register CopySrcReg;
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arsenm wrote:

<!--__GRAPHITE_HTML_TAG_START__--><p class='graphite__hidden'><i>[Re: line +634]</i></p><!--__GRAPHITE_HTML_TAG_END__-->
The case down here is also a problem, but needs a separate test
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https://github.com/llvm/llvm-project/pull/216598