[llvm] [PHIElimination] Reuse existing COPY in predecessor basic block (PR #131837)

[Mikael Holmén via llvm-commits]

mikaelholmen wrote:

> Thanks for looking into this, issued a fix in: #146320.

Thanks, that fix seems to solve that problem.

It looks like there are other problems as well though. I don't have a reproducer I can share now but if we have virtual registers of two different register classes "32BitRC" with 32 bit registers and "16BitRC" with 16 bit registers it looks like it turns
```
  %8:32BitRC = COPY %7:32BitRC
  [...]
  %2:16BitRC = PHI %8.low16:32BitRC, %bb.0, %1:16BitRC, %bb.1
```
into
```
  %9:16BitRC = COPY %7:32BitRC
  [...]
  %2:16BitRC = COPY killed %9:16BitRC
```
i.e. it ignores the sub register in the PHI?



https://github.com/llvm/llvm-project/pull/131837