[llvm] [RISCV] Fix coalesced vsetvli's AVL LiveInterval not always being shrunk (PR #98286)

[Philip Reames via llvm-commits]

https://github.com/preames approved this pull request.

LGTM to the current patch.

I talked with Luke offline, and was convinced this is correct as structured.  I'm not a fan of the style - doing the updates in two places, and out of order - but I'm explicitly okay with this landing, and I will post an NFC tomorrow.

https://github.com/llvm/llvm-project/pull/98286