[llvm] [LV] NFCI: Create VPExpressions in transformToPartialReductions. (PR #182863)

[Benjamin Maxwell via llvm-commits]

https://github.com/MacDue approved this pull request.

This LGTM, but I think some of the logic in `tryToMatchAndCreateMulAccumulateReduction` (and elsewhere) may now be untested (and possibly can be removed). Several chunks of it were added explicitly for partial reductions. For example, this section was added with `fadd` partial reductions: https://github.com/llvm/llvm-project/blob/a72f7fcc92fee0fb6f1cfaa45d4d75dda43d2280/llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp#L4521-L4539 (removing it does not break any tests now). 

https://github.com/llvm/llvm-project/pull/182863