When searching for non-null, check the dominator tree.

When searching for non-null, check the dominator tree.

The non-null bitmap only indicates which blocks non-null setting occurs.
Generalized queries need to search the dom tree, whereas propagation
engines only need to know the current block.  Add a flag for this purpose.

	* gimple-range-cache.cc (non_null_ref::non_null_deref_p): Search
	dominator tree is available and requested.
	(ranger_cache::ssa_range_in_bb): Don't search dom tree here.
	(ranger_cache::fill_block_cache): Don't search dom tree here either.
	* gimple-range-cache.h (non_null_deref_p): Add dom_search param.