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

[Mikael Holmén via llvm-commits]

mikaelholmen wrote:

> It looks like there are other problems as well though. I don't have a reproducer I can share now

I fiddled with the repro for my out-of-tree target and changed it to something for aarch64:
```llc bbi-108462_2_aarch64.mir -mtriple=aarch64 -o - -run-pass phi-node-elimination```

Now, I don't know aarch64 and its register classes but the sub register access "%1.sub_32" is just dropped in the output.
For my target at least, that is wrong because in the case where it originally broke, the sub registers specifies if we should use the high or low 16 bits of the 32 bit operand.

[bbi-108462_2_aarch64.mir.gz](https://github.com/user-attachments/files/20978409/bbi-108462_2_aarch64.mir.gz)


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