[InstCombine] Intrinsic::vector_partial_reduce_add constant folding special cases

As discussed here: https://github.com/llvm/llvm-project/pull/212112#pullrequestreview-5238321505

> Noting a couple ideas for follow ups:
> 1. We could handle scalable idioms such as all zeros, splats.
> 2. We could have an instcombine generalization for when Acc isn't a constant, but Vec is to reduce the constant, and replace the partial reduce with a plain add. (Said differently, view a partial.reduce.add(Acc, C) as add(Acc, partial.reduce.add(0, C)) and apply the constant folding idea from here.)
> 3. The prior item would also generalize to various non-constant Vec cases.

Start with the linked pull request and review, then trace the existing InstCombine handling for Intrinsic::vector_partial_reduce_add. Clarify which of the three proposed follow-ups is in scope and how each case should be validated; the work is done when the chosen constant-folding or generalization behavior is implemented and verified.

Written by the indexing model from the issue text.