tree-optimization/28614 - high FRE time for gcc.c-torture/compile/20001226-1.c

tree-optimization/28614 - high FRE time for gcc.c-torture/compile/20001226-1.c

I noticed that for gcc.c-torture/compile/20001226-1.c even -O1 has
around 50% of the compile-time accounted to FRE.  That's because
we have blocks with a high incoming edge count and
can_track_predicate_on_edge visits all of them even though it could
stop after the second.  The function is also called repeatedly for
the same edge.  The following fixes this and reduces the FRE time
to 1% on the testcase.

	PR tree-optimization/28614
	* tree-ssa-sccvn.cc (can_track_predicate_on_edge): Avoid
	walking all edges in most cases.
	(vn_nary_op_insert_pieces_predicated): Avoid repeated
	calls to can_track_predicate_on_edge unless checking is
	enabled.
	(process_bb): Instead call it once here for each edge
	we register possibly multiple predicates on.