Checking if a type is a reference must be gsharing aware.

Checking if a type is a reference must be gsharing aware.

	*  mini-generic-sharing.c: Add mini_type_is_reference
	that returns true for VAR and MVAR if compiling under
	gsharing.

	* method-to-ir.c: Replace usage of mono_type_is_reference
	with mini_type_is_reference.

	* mini-gc.c: Ditto.

	* mini.c: Ditto.

	With the old behavior, the JIT could interpret a (M)VAR
	variable and not been a reference type. This is an issue
	for sgen as we could forgot to emit some write barriers
	or generate the proper stack map.