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

[Amara Emerson via llvm-commits]

aemerson wrote:

However, I just noticed that this is 2 separate commits in a PR. I believe current PR policy is one change per PR, with follow up commits used for addressing review feedback.

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