[Test] Add tests showing congruent IVs not removed by IndVars

In the added cases we have two congruent IVs. IndVars widens at least one of them.
If they are both widened, then one of them is erased as they stay congruent after
widening. However if only one IV is widened, the other one stays in the loop.
We can simply erase the narrow IV and replace its uses with truncates of the
widest IV.