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

[Min-Yih Hsu via llvm-commits]

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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