[llvm] [RISCV] Use vleff's AVL when output VL doesn't dominate in RISCVVLOptimizer (PR #156618)

[Luke Lau via llvm-commits]

lukel97 wrote:

> As a follow up, you could consider the same kind of logic as ensureDominates in RISCVVectorPeephole, or maybe rematerialization.

Is this in reference to #156639?

https://github.com/llvm/llvm-project/pull/156618