Add gsharing support for cached casts.

Add gsharing support for cached casts.

	* class-internals.h: Add new kind of rgctx slot.

	* method-to-ir.c (handle_castclass): Emit gsharing code
	for cast with caching.

	* method-to-ir.c (handle_isinst):

	* mini-generic-sharing.c: Implement MONO_RGCTX_INFO_CAST_CACHE
	slot type. It's a 2 word domain allocated memory block with the
	cache on the first word and the vtable on the second.

	This design does a single rgctx fetch instead of two. It's
	a bit faster and can potentialy save some slots.