Fix typing on generated LXV2DX/STXV2DX instructions

[PPC] Previously when casting generic loads to LXV2DX/ST instructions we
would leave the original load return type in place allowing for an
assertion failure when we merge two equivalent LXV2DX nodes with
different types.

This fixes PR27350.

Reviewers: nemanjai

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D19133

llvm-svn: 266438