[llvm] [RISCV] Introduce VLOptimizer pass (PR #108640)

[Luke Lau via llvm-commits]

lukel97 wrote:

> If everyone is on board for this approach, I think that would be good. But I want to highlight here that I think the test coverage is not in a place where I'd feel comfortable to enabling this pass by default. I think this is a must have before turning it on.

I would be happy to iterate in tree as well, we can leave it off by default and add some .mir tests. As for moving getOperandInfo out into TableGen, it was mostly a refactoring idea and I don't think we should block this PR on it. I can take a stab at it this week in parallel and see if it's feasible or not.

https://github.com/llvm/llvm-project/pull/108640