Fix incorrect use of VM_CALL_KW_SPLAT_MUT in zsuper with keyword splat

	For zsuper calls with a keyword splat but no actual keywords, the
	keyword splat is passed directly, so it cannot be mutable, because
	if the callee accepts a keyword splat, changes to the keyword splat
	by the callee would be reflected in the caller.

	While here, simplify the logic when the method supports
	literal keywords.  I don't think it is possible for
	a method with has_kw param flags to not have keywords, so add an
	assertion for that, and set VM_CALL_KW_SPLAT_MUT in a single place.