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

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

> This does not look particularly compelling to me. It's a regression on average, and also duplicates a lot of logic.

Perhaps it is possible to strip down the complicated logic, obtaining a cheaper estimate, and reducing the logic-duplication. I will benchmark it on the top regressions, and see what we can strip out. 

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