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

[Michael Maitland via llvm-commits]

michaelmaitland wrote:

I have forced push because there had not been any review and I was missing some important bug fixes. I also opted to remove some functionality that I think would be better as a follow up patch to make the review here more straightforward.

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