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

[Sander de Smalen via llvm-commits]

sdesmalen-arm wrote:

> 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.

Good point. Let me know if you want me to remove that in this PR, or in a follow up.

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