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

[Guy David via llvm-commits]

guy-david wrote:

Rebased. Two tests are failing but are unrelated:
```
Failed Tests (2):
  LLVM :: CodeGen/ARM/constant-islands-cfg.mir
  LLVM :: CodeGen/Thumb/2009-08-12-ConstIslandAssert.ll
```

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