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

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

I'm also running into miscompilations caused by this in libvpx, for armv7 targets, with the latest git main of llvm.

The reproducer for the miscompile is https://martin.st/temp/y4minput-preproc.c, compiled like this:
```console
$ clang -target armv7-w64-mingw32 y4minput-preproc.c -c -o y4minput.c.o -O2
```

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