Scalar to vector conversions using direct moves

This patch corresponds to review:
http://reviews.llvm.org/D11471

It improves the code generated for converting a scalar to a vector value. With
direct moves from GPRs to VSRs, we no longer require expensive stack operations
for this. Subsequent patches will handle the reverse case and more general
operations between vectors and their scalar elements.

llvm-svn: 244921