Use dominators to reduce cache-flling.

Before walking the CFG and filling all cache entries, check if the
same information is available in a dominator.

	* gimple-range-cache.cc (ranger_cache::fill_block_cache): Check for
	a range from dominators before filling the cache.
	(ranger_cache::range_from_dom): New.
	* gimple-range-cache.h (ranger_cache::range_from_dom): Add prototype.