[MIPS GlobalISel] Select MSA vector generic and builtin add

Select vector G_ADD for MIPS32 with MSA. We have to set bank
for vector operands to fprb and selectImpl will do the rest.
__builtin_msa_addv_<format> will be transformed into G_ADD
in legalizeIntrinsic and selected in the same way.
__builtin_msa_addvi_<format> will be directly selected into
ADDVI_<format> in legalizeIntrinsic. MIR tests for it have
unnecessary additional copies. Capture current state of tests
with run-pass=legalizer with a test in test/CodeGen/MIR/Mips.

Differential Revision: https://reviews.llvm.org/D68984

llvm-svn: 375501