[RISCV] Prune dead LI in vsetvli coalescing with trivially dead vsetvli (#98647)

This is a follow up to ff8a03a7. On the review for that, I'd suggested a
stylistic rework, but after discussion we decided to move forward with
the fix as it was. This change is a small part of that suggested rework.
Once I sat down and wrote the code, I think I've convinced myself of an
entirely different approach (tbd), but for the moment, let's use a
lambda to share code so that we can pickup a missed optimization, and
reduce some duplication.

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Co-authored-by: Luke Lau <luke_lau@icloud.com>

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