[AArch64][GlobalISel] When copying from a gpr32 to an fpr16 reg, convert to fpr32 first.

This is a follow on commit to r[x] where we fix the other direction of copy.
For this case, after converting the source from gpr32 -> fpr32, we use a
subregister copy, which is essentially what EXTRACT_SUBREG does in SDAG land.

https://reviews.llvm.org/D43444

llvm-svn: 325550