[llvm] [RISCV] Teach RISCVInsertVSETVLI to work without LiveIntervals (PR #94686)

[Philip Reames via llvm-commits]

preames wrote:

@lukel97 I just had a thought.  We still have MRI after vector regalloc, and while we can't be guaranteed to find a unique definition, MRI does have means for cheaply getting a unique definition if one exists.  You see any reason why we can't opportunistically use that in hasNonZeroAVL, and hasSameAVL?  We couldn't easily use it during state merging, but off the top of my head I'm not seeing any reason that compatibility style reasoning can't?  

We don't necessarily need to add that in this patch - mostly thinking about mitigations for perf loss if anyone does care about O0 intrinsic performance.  

https://github.com/llvm/llvm-project/pull/94686