[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

llvm-svn: 277219