Improve handling of ONE-VALUE-AST-MIXIN in a context with no result.

We take advantage of the newly introduced predicate
SIDE-EFFECT-FREE-P.  If a ONE-VALUE-AST-MIXIN is compiled in a context
with a single successor and that requires no results, we check whether
it is side-effect free.  If so, we warn and generate no code.
Otherwise, we CALL-NEXT-METHOD with a temporary variable.