[llvm] MIPS: Support isLegalICmpImmediate and isLegalAddImmediate (PR #133400)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

Testcase?  I'd expect that implementing these functions has some effect on generated code, even if you're not explicitly using them in the MIPS backend.

https://github.com/llvm/llvm-project/pull/133400