Add a merge_range to ssa_cache and use it. add empty_p and param tweaks.

	* gimple-range-cache.cc (ssa_cache::merge_range): New.
	(ssa_lazy_cache::merge_range): New.
	* gimple-range-cache.h (class ssa_cache): Adjust protoypes.
	(class ssa_lazy_cache): Ditto.
	* gimple-range.cc (assume_query::calculate_op): Use merge_range.