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

[Yingwei Zheng via llvm-commits]

dtcxzyw wrote:

> The tests have been split out to a separate commit, but will get squashed on merge. Could you do a separate PR for the tests? Or commit them directly with the suggested adjustment?

Done. https://github.com/llvm/llvm-project/pull/76512

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