[llvm] ConstraintElim: add dry-run routine to fail early (PR #99670)

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

> I also wonder if this suffices to establish whether the early conservative estimation of rows/columns outweighs the somewhat noticeable code duplication, maybe worth it to run SPEC with this before further reviewing?

Certainly. Unfortunately, I don't have access to a machine with SPEC set up. Perhaps someone else has access to one?

I think it might be possible to get rid of at least some of the code duplication using a template parameter. Also, I think the regressions are coming from tiny compilation units, for which I think we can short-circuit the dry-run using a rough estimate like number of items in the worklist.

https://github.com/llvm/llvm-project/pull/99670