Do not lift sequence from right-hand side of binary operation.  Fix #343
v2.4.3
Better fix for #343

We can in fact lift sequences, but only if the operation is assignment and
the left-hand side has no side effects nor property access -- that should
guarantee that whatever we place before it cannot affect the sense of the
assignment.

Dropped contrived test case (too hard to support it now), added a more
meaningful one.