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

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dong-miao wrote:

> @dong-miao why was this closed? Were the changes merged separately?

I don't know why this pull request cannot be merged. It seems that another reviewer did not approve it, but he did not provide a corresponding reason or solution.This commit has been put on hold here without any new progress.

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