[llvm] [RISCV] Move RISCVInsertVSETVLI to after phi elimination (PR #91440)

[Philip Reames via llvm-commits]

https://github.com/preames commented:

None of the remaining test diffs are concerning, here's a pass of minor comments over the code.

I want to be careful to say that I don't have enough depth on the LiveIntervals/VNInfo stuff to be a knowledgeable reviewer.  My comments are mostly surface level.  

https://github.com/llvm/llvm-project/pull/91440