[clang] [llvm] [MIPS] Add FeatureMSA to i6400 and i6500 cores (PR #134985)

[Min-Yih Hsu via cfe-commits]

================

----------------
mshockwave wrote:

I just noticed there is already `test/CodeGen/Mips/msa/arithmetic.ll` (which this file is probably based on), could add additional `RUN` lines to that file instead, in order to reuse it?

https://github.com/llvm/llvm-project/pull/134985