mark region also for USE predicate discovery

mark region also for USE predicate discovery

The following makes sure to mark the dominating region also for
USE predicate discovery, avoiding compute_control_dep_chain to
walk to unrelated areas, eating up walking budget.

	* gimple-predicate-analysis.cc (dfs_mark_dominating_region):
	Adjust to take the region exit source as argument.
	(uninit_analysis::init_from_phi_def): Adjust.
	(uninit_analysis::init_use_preds): Mark the dominating region
	before computing control dependences.