fixReductionScalarResumeWhenVectorizingEpilog updates the resume phis in
the scalar preheader. Instead of looking at all recipes in the middle
block and finding their resume-phi users we can iterate over all resume
phis in the scalar preheader directly.
This slightly simplifies the code and removes the need to look for the
resume phi.
Also slightly simplifies https://github.com/llvm/llvm-project/pull/141860.
4a6d31f4bf