Select vector G_SUB for MIPS32 with MSA. We have to set bank
for vector operands to fprb and selectImpl will do the rest.
__builtin_msa_subv_<format> will be transformed into G_SUB
in legalizeIntrinsic and selected in the same way.
__builtin_msa_subvi_<format> will be directly selected into
SUBVI_<format> in legalizeIntrinsic.
Differential Revision: https://reviews.llvm.org/D69306
Petar Avramovic