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

[Philip Reames via llvm-commits]

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

LGTM

As discussed in today's sync call, we have consensus to land this in tree (off by default!) and continue iterating.  There is a good amount of code sharing, and improved testing which needs to happen before this can be enabled by default.  All parties are aware, and enabling that incrementally is the goal of landing now.  

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