[llvm] [RISCV] Don't forward AVL in VSETVLIInfo if it would clobber other definitions (PR #97264)

[Philip Reames via llvm-commits]

preames wrote:

If I'm following the logic of this patch, don't we need to adjust the equally-zero reasoning in transferBefore as well?  That extends the live range of an AVL too doesn't it?

If so, I think we're also missing an assert in insertVSETVLI that the AVL we're trying to extend has a single def.  

https://github.com/llvm/llvm-project/pull/97264