[llvm] [ConstraintElimination] Add eq/ne facts to signed constraint system (PR #121423)

[Stephen Senran Zhang via llvm-commits]

zsrkmyn wrote:

> Compile-time: http://llvm-compile-time-tracker.com/compare.php?from=193ea83dd7e879ddd4e3dfb1fa74a676b528e4a6&to=b4b2d9643dfef63550aecaea008ece92c549247f&stat=instructions:u

I'm thinking if we can only add facts to signed system only for eq, but not ne, at line 1598, to avoid unnecessary `getConstraint` & `decompose` for ne cases to save compiling time, as ne cannot be handled for signed system for now.

https://github.com/llvm/llvm-project/pull/121423