[SCCP] Simplify general icmp instructions

[Camsyn]

If we know X ∈ R1, the range check x ∈ R2 can be relaxed into X ∈ R1 ∩ R2 or X ∈ R2 ∪ Inverse(R1).

The relaxed one may be more efficient if we can represent it with one icmp eq, which could motivate more optimization like SimplifyCFG.

This optimization is a more generic version of InstCombine's foldICmpWithDominatingICmp, which only works on those CmpBB with ONE DomBB, i.e., cannot handle the following case:

if (x == 0) return false;
if (x == 1) return false;
// Could be simplified as x == 2;
return x <u 3);

Alive2 Proof : https://alive2.llvm.org/ce/z/HMgkuu