[Power9] Add exploitation of non-permuting memory ops

[Power9] Add exploitation of non-permuting memory ops

This patch corresponds to review:
https://reviews.llvm.org/D19825

The new lxvx/stxvx instructions do not require the swaps to line the elements
up correctly. In order to select them over the lxvd2x/lxvw4x instructions which
require swaps, the patterns for the old instruction have a predicate that
ensures they won't be selected on Power9 and newer CPUs.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@282143 91177308-0d34-0410-b5e6-96231b3b80d8