[ConstraintElimination] Remove ConstraintListTy (NFCI).

This patch simplifies constraint handling by removing the
ConstraintListTy wrapper struct and moving the Preconditions directly
into ConstraintTy. This reduces the amount of memory needed for managing
constraints.

The only use case for ConstraintListTy was adding 2 constraints to model
ICMP_EQ conditions. But this can be handled by adding an IsEq flag. When
adding an equality constraint, we need to add the constraint and the
inverted constraint.