[llvm] [ConstraintElim] Drop invalid rows instead of failing the elimination (PR #76299)

[Florian Hahn via llvm-commits]

fhahn wrote:

Intuitively this makes sense, as dropping information/rows should only ever turn an unsatisfiable system into a satisfiable system, but not the other way around.

I'd still like to manually check the steps, as we may drop rows we created to eliminate a variable.

https://github.com/llvm/llvm-project/pull/76299