[llvm] [X86][CodeGen] Support partial-reduce dot products (PR #205373)

[Simon Pilgrim via llvm-commits]

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

LGTM - as a first step. I think we're missing a number of opportunities to properly use pmaddwd/pmaddubsw in a baseline implementation but we can investigate that later.

https://github.com/llvm/llvm-project/pull/205373