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

[Philip Reames via llvm-commits]

preames wrote:

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

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