[AArch64][RegisterBankInfo] Add mapping for G_FPEXT.

This fixes http://llvm.org/PR32560. We were missing a description for
half floating point type and as a result were using the FPR 32 mapping.
Because of the size mismatch the generic code was complaining that the
default mapping is not appropriate. Fix the mapping description so that
the default mapping can be properly applied.

llvm-svn: 317287