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

[Philip Reames via llvm-commits]

preames wrote:

As another possible approach to this problem... could we create a new virtual register for the forwarded AVL, use a COPY from the existing range (within it's existing live range), and then a use of the new virtreg at the use point?  (Thinking specifically in the case we can't extend the existing use.)  This would avoid the need to detect the "illegal to forward" case early.

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