[llvm] [RISCV][VLOPT] Compute demanded VLs up front (PR #124530)

[Philip Reames via llvm-commits]

preames wrote:

> I've directly committed [52c1162](https://github.com/llvm/llvm-project/commit/52c116218b61c088ac77f26c7b57347a5f54224d) to fix the fact that `DemandedVLs` is never cleared.

Thanks.  I really should have caught that in review - oops.  

https://github.com/llvm/llvm-project/pull/124530