[MIPS GlobalISel] Select MSA vector generic and builtin mul

Select vector G_MUL for MIPS32 with MSA. We have to set bank
for vector operands to fprb and selectImpl will do the rest.
Manual selection of G_MUL is now done for gprb only.
__builtin_msa_mulv_<format> will be transformed into G_MUL
in legalizeIntrinsic and selected in the same way.

Differential Revision: https://reviews.llvm.org/D69310