Improve optimization of ASan checks.

Improve optimization of ASan checks.

2014-12-03  Yury Gribov  <y.gribov@samsung.com>

gcc/
	* sanopt.c (maybe_get_single_definition): New function.
	(maybe_get_dominating_check): Ditto.
	(can_remove_asan_check): Ditto.
	(struct tree_map_traits): New struct.
	(struct sanopt_ctx): Use custom traits for asan_check_map.
	(maybe_optimize_ubsan_null_ifn): Move code to
	maybe_get_dominating_check.
	(maybe_optimize_asan_check_ifn): Move code and take non-SSA expressions
	into account when optimizing.
	(sanopt_optimize_walker): Optimize ASan checks even when
	recovering.

From-SVN: r218304