[llvm] [RISCV] Let LiveIntervals::shrinkToUses compute dead AVLs (PR #90629)

[Luke Lau via llvm-commits]

lukel97 wrote:

The issue reported in #95865 was caused by us forgetting to remove the dead AVL from the LiveIntervals instruction map, but it turns out this PR fixes this by reusing the same code path for the dead AVL and vsetvli cc @patrick-rivos 

https://github.com/llvm/llvm-project/pull/90629