[llvm] LoopVectorize: fix phi cost when it is scalar after vectorization (PR #74456)

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

Thanks @fhahn: the patch is now ready. Your comment about replacing the `load` with the `store` makes sense, but can you clarify why you thought `-force-vector-width=4` would work? Also, if you know, what is the difference between sse and sse4.2 in the context of this test?

https://github.com/llvm/llvm-project/pull/74456