Bitcasts between FP and INT values using direct moves

This patch corresponds to review:
http://reviews.llvm.org/D15286

LLVM IR frequently contains bitcast operations between floating point and
integer values of the same width. Doing this through memory operations is
quite expensive on PPC. This patch allows the use of direct register moves
between FPRs and GPRs for lowering bitcasts.

llvm-svn: 255246