When a multiply expression is IVL_VT_REAL, then the default width is
the width of the operands, and not the sum of the operands. In fact, it
should turn out to be 1.
Also, the width of the multiply expression when we get to the ivl_target
API, need not be the sum of the widths of its arguments. In fact, if the
arguments are signed, that would be completely wrong. So adjust the
stub target to properly test this constraint.