[X86] Match PSADBW in straight-line code

[X86] Match PSADBW in straight-line code

Up until now, we only had code to match PSADBW patterns that look like what
comes out of the loop vectorizer - a partial reduction inside the loop body
that gets fed into a horizontal operation in a different basic block.

This adds support for straight-line patterns, like those generated by the
SLP vectorizer.

Differential Revision: https://reviews.llvm.org/D22889

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@277219 91177308-0d34-0410-b5e6-96231b3b80d8