[flang] Prevent bad expression rewrite 0*ARR -> 0 (#79853)

Don't rewrite 0*X to 0 if X is not scalar. Up until now this hasn't
shown up as a bug because a scalar 0 works in nearly all expressions
where an array would be expected. But not in all cases -- this bad
rewrite can cause generic procedure resolution to fail when it causes an
actual argument to have an unsupported rank.