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

[Luke Lau via llvm-commits]

lukel97 wrote:

> The initial demanded VL is sound, but not precise. As you reduce a transitive user, you can reduce the VL of an instruction which in turn reduces it of that instructions defs. You backward walk achieves this within a basic block, but we loose the cross block case.

It should be possible to 

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