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

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon requested changes to this pull request.

This still needs a lot of cleanup - its barely readable in its current form.

Merge as many of the vector-partial-reduce test files as you can - I don't see the purpose of the vex split separations 

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