[InstSimplify] Fold icmp with dominating assume

If we assume(x > y), then we should be able to fold the basic
implications of that, like x >= y. This already happens if either
one of the operands is constant (LVI) or if the conditions are
exactly the same (GVN), but not if we have an implication with
non-constant operands. Support this by querying AssumptionCache.

Fixes https://bugs.llvm.org/show_bug.cgi?id=40149.

Differential Revision: https://reviews.llvm.org/D82717