[Eva] Improves the precision of the symbolic locations domain.

Changes the criterion according to which the symbolic domain retains the value
of an expression:
- if the expression is an lvalue with an imprecise location;
- if the expression is a binary operation between two expressions, each
  containing an lvalue with an imprecise (non-singleton) value.

Otherwise, the value should always be inferred by the cvalue domain, or can
be precisely computed from values inferred by the cvalue domain.