Avoid recomputing PHI results after failure (PR104203).

Avoid recomputing PHI results after failure (PR104203).

Resolves:
PR tree-optimization/104203 - huge compile-time regression in pointer_query

gcc/ChangeLog:

	PR tree-optimization/104203
	* gimple-ssa-warn-access.cc (pass_data pass_data_waccess): Use
	TV_WARN_ACCESS.
	* pointer-query.cc (access_ref::merge_ref): Change return type.
	Convert failure to a conservative success.
	(access_ref::get_ref): Adjust to the change above.  Short-circuit
	PHI evaluation after first failure turned into conservative success.
	* pointer-query.h (access_ref::merge_ref): Change return type.
	* timevar.def (TV_WARN_ACCESS): New timer variable.