[x86] avoid vector load narrowing with extracted store uses (PR42305)

This is an exception to the rule that we should prefer xmm ops to ymm ops.
As shown in PR42305:
https://bugs.llvm.org/show_bug.cgi?id=42305
...the store folding opportunity with vextractf128 may result in better
perf by reducing the instruction count.

Differential Revision: https://reviews.llvm.org/D63517

llvm-svn: 363853