[llvm] [RISCV] Add sink-and-fold support for RISC-V. (PR #67602)

[Alex Bradbury via llvm-commits]

https://github.com/asb approved this pull request.

LGTM.

(Question for future iterations, rather than this patch): Do we ever get an ORI we'd want to fold, due to DAGCombiner converting a+b to a|b when that's a legal transformation?

https://github.com/llvm/llvm-project/pull/67602