[PowerPC] Remove redundant direct moves when extracting integers and converting to FP

This patch corresponds to review:
https://reviews.llvm.org/D21354

We use direct moves for extracting integer elements from vectors. We also use
direct moves when converting integers to FP. When these operations are chained,
we get a direct move out of a VSR followed by a direct move back into a VSR.
These are redundant - all we need to do is line up the element and convert.

llvm-svn: 275796