Add VSX Scalar loads and stores to the PPC back end

Add VSX Scalar loads and stores to the PPC back end

This patch corresponds to review:
http://reviews.llvm.org/D9440

It adds a new register class to the PPC back end to contain single precision
values in VSX registers. Additionally, it adds scalar loads and stores for
VSX registers.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@236755 91177308-0d34-0410-b5e6-96231b3b80d8