[llvm] [RISCV] Implement RISCVISD::SHL_ADD and move patterns into combine (PR #89263)

[Pengcheng Wang via llvm-commits]

wangpc-pp wrote:

Sorry for bothering, but I have to say that this account (@pcwang-thead) is not used and I can't access it now. Hopefully its access can be revoked after six months (https://discourse.llvm.org/t/rfc2-new-criteria-for-commit-access/77110), or @tstellar can just remove it.

https://github.com/llvm/llvm-project/pull/89263