Add direct moves to/from VSR and exploit them for FP/INT conversions

Add direct moves to/from VSR and exploit them for FP/INT conversions

This patch corresponds to review:
http://reviews.llvm.org/D8928

It adds direct move instructions to/from VSX registers to GPR's. These are
exploited for FP <-> INT conversions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@234682 91177308-0d34-0410-b5e6-96231b3b80d8