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

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

Okay, so it's a near-neutral change overall, with huge improvements in a some cases. I don't think it's possible to optimize this further, without duplicating decompose(), and that is probably undesirable.

Although the benefits of this patch on its own are questionable, it is a prerequisite for another neutral change: the change to the Matrix data structure. The change to the Matrix data structure is a prerequisite for building more complicated things on top of ConsttraintSystem: I'm planning to write a solver-based dependence analysis.

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