[llvm] [RISCV][GISel] Add initial pre-legalizer combiners copying from AArch64. (PR #65663)

[Amara Emerson via llvm-commits]

https://github.com/aemerson commented:

Seems reasonable to me from a GISel view, I'll let RISCV maintainers give the approval.

https://github.com/llvm/llvm-project/pull/65663