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

[Min-Yih Hsu via llvm-commits]

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+; Test the MSA intrinsics that are encoded with the SPECIAL instruction format.
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mshockwave wrote:

Could we use `update_llc_test_checks.py` to maintain this test?

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