[SLP] Allow vectorization of reversed loads.

Summary:
Reversed loads are handled as gathering. But we can just reshuffle
these values. Patch adds support for vectorization of reversed loads.

Reviewers: RKSimon, spatel, mkuper, hfinkel

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D43022

llvm-svn: 325134