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

[Luke Lau via llvm-commits]

lukel97 wrote:

I've undone all the std::optional API changes and stacked this on top of #124734. The analysis should now also be precise across blocks as it's computed globally in post-order traversal, thanks for catching that.

The diff should be much smaller now, I think the initial version made the change look a lot larger than what it is. 

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