[Eva] Fixes the automatic loop unrolling.

If an lvalue [lv1] from a loop exit condition is always assigned to the value of
another lvalue [lv2] in the loop body, then the heuristic uses [lv2] as loop
variant instead of [lv1].
This commit ensures that this replacement is only done if [lv1] is assigned
to the value [lv2] in all possible path of the loop body.