[llvm] promote Pseduo Opcode from 32bit to 64bits after eliminating the `extsw` instruction in PPCMIPeepholes optimization (PR #85451)

[Amy Kwan via llvm-commits]

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@@ -1051,6 +1051,7 @@ bool PPCMIPeephole::simplifyCode() {
                    TII->isSignExtended(NarrowReg, MRI)) {
           // We can eliminate EXTSW if the input is known to be already
           // sign-extended.
+          TII->replaceInstrAfterElimExt32To64(NarrowReg, MRI, 0, LV);
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amy-kwan wrote:

A couple comments regarding this line:

- This call does not make sense with the comment on 1052-1053. It would be better to put this call above the comment.
- The naming of the function seems unclear. We can consider renaming the function from `replaceInstrAfter` to `promoteInstrBefore`, or something similar.
- Add new documentation for the function.

https://github.com/llvm/llvm-project/pull/85451