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

[Luke Lau via llvm-commits]

lukel97 wrote:

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

Thanks for fixing that, I think we've previously had issues with forgetting to clear structures in passes before

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