[DAGCombine] matchBinOpReduction - add partial reduction matching

This patch adds support for recognizing cases where a larger vector type is being used to reduce just the elements in the lower subvector:

e.g. <8 x i32> reduction pattern in a <16 x i32> vector:

<4,5,6,7,u,u,u,u,u,u,u,u,u,u,u,u>
<2,3,u,u,u,u,u,u,u,u,u,u,u,u,u,u>
<1,u,u,u,u,u,u,u,u,u,u,u,u,u,u,u>

matchBinOpReduction returns the lower extracted subvector in such cases, assuming isExtractSubvectorCheap accepts the extraction.

I've only enabled it for X86 reduction sums so far. I intend to enable it for the bitop/minmax cases in future patches, and eventually I think its worth turning it on all the time. This is mainly just a case of ensuring calls to matchBinOpReduction don't make assumptions on the vector width based on the original vector extraction.

Fixes the x86 partial reduction sum cases in PR33758 and PR42023.

Differential Revision: https://reviews.llvm.org/D65047

llvm-svn: 366933