Fix performance issue: non-trivial invariants were pruned one at a time
Fix performance issue: non-trivial invariants were pruned one at a time

In commit 6954967d non-trivial invariants are processed individually
so that they are taken into account to prune next candidates.
However, commit 79fea641 stopped adding discovered invariants to pruner
so that no information is learnt and each non-trivial invariant is
discovered multiple times.

kind2-mc/kind2,daniel-larraz/kind2