[llvm] [RISCV] Introduce VLOptimizer pass (PR #108640)

[Philip Reames via llvm-commits]

https://github.com/preames commented:

A couple of detail comments inline, but I want to focus on high level review strategy.

Looking at the code for this, I spot a couple possible bugs (flagged in inline comments), but nothing really terrible code structure wise.  I'd like to propose that we defer most of the code cleanup suggestions until after this is in tree.  Specifically, I'm proposing the following:
1) This patch is repurposed to add the transform pass off-by-default.  We subset as needed to get something correct in tree with minimal delay.
2) We iterate in tree to common code, and to add back any key features subset in step (1).  
3) We enable the pass in it's own commit.  

I'm very hesitant to do the usual style review here because there's so much room for subtle bugs to be introduced, and if we get into such a cycle the review could extend nearly indefinitely.  I'm willing to assume good faith follow up here, and frankly will likely invest a bunch of time myself in getting this code into long term supportable state. 

https://github.com/llvm/llvm-project/pull/108640