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

[Antonio Frighetto via llvm-commits]

antoniofrighetto wrote:

> Managed to eek out another small optimization: http://llvm-compile-time-tracker.com/compare.php?from=c5ee3c05ca61f3fae11337c5853aee7b450a9dc6&to=f0d674c9f5aa601c0e5472f208ff96f3d855d9b6&stat=instructions%3Au.
> 
> @dtcxzyw Could you kindly re-check against the latest commit?

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?

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