[llvm] [RISCV] Support [mh]edelegh (PR #121634)

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dong-miao wrote:

> The commit message is rather lacking specifics; especially "[RISCV]Update CSRs" is (a) badly formatted (b) too terse and vague. Since this is just for RV32 [mh]edelegh CSRs, it would be appropriate to mention that explicitly.
Do this pull request require any additional changes?


https://github.com/llvm/llvm-project/pull/121634