[llvm] [WIP][AMDGPU] Fix emitting illegal COPY (PR #131752)

[Shilei Tian via llvm-commits]

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@@ -899,6 +899,55 @@ bool SIFixSGPRCopies::lowerSpecialCase(MachineInstr &MI,
                                       MI, MI.getDebugLoc())) {
       I = std::next(I);
       MI.eraseFromParent();
+    } else {
+      // At this point, if we still have a VGPR → SGPR copy, it is completely
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shiltian wrote:

Not a good idea to use `→` in the comment. `->` is preferred.

https://github.com/llvm/llvm-project/pull/131752