Fix constant-needs-allocation? for $values uses

Fix constant-needs-allocation? for $values uses

* module/language/cps/dfg.scm (constant-needs-allocation?): Use of a
  constant in a $values expression of any arity does not cause slot
  allocation.

* module/language/cps/compile-bytecode.scm (compile-fun): Allow $values
  with a constant value to be compiled in test context.  Really we
  should fold these in a previous pass!