Bitcasts between FP and INT values using direct moves

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

This patch was meant to land in revision 255246, but I accidentally uploaded
the patch that corresponds to http://reviews.llvm.org/D15372 in that revision
accidentally.

Thereby, this patch is the actual Bitcasts using direct moves patch, whereas
http://reviews.llvm.org/rL255246 actually corresponds to
http://reviews.llvm.org/D15372.

llvm-svn: 255649