[RISCV][VLOpt] Reorganize visit order and worklist management (#123973)

This implements a suggestion by Craig in PR #123878. We can move the
worklist management out of the per-instruction work and do it once at
the end of scanning all the instructions. This should reduce repeat
visitation of the same instruction when no changes can be made.

Note that this does not remove the inherent O(N^2) in the algorithm.
We're still potentially visiiting every user of every def.

I also included a guard for unreachable blocks since that had been
mentioned as a possible cause. It seems we've rulled that out, but
guarding for this case is still a good idea.

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