[RISCV][GISel] Add initial pre-legalizer combiners copying from AArch64.

[topperc]

• [RISCV][GlobalISel] Select ALU GPR instructions
• [RISCV][GISel] Add initial pre-legalizer combiners copying from AArch64.

This is stack on https://reviews.llvm.org/D76445 to get the RISCVGISel.td and the ability to select basic instructions.

[aemerson]

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

[aemerson]

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.

[topperc]

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.

It is only because I didn't know how to handle this commit being dependent on another review.

[aemerson]

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.

It is only because I didn't know how to handle this commit being dependent on another review.

Join the club, we had jackets made.

So far there's no known way to have stacked reviews 🙃

[preames]

LGTM from the RISCV side. Relying heavily on @aemerson previous review for the GISEL bits.

[topperc]

Landed as 8677aaa