tree-optimization/108353 - copyprop iteration order

tree-optimization/108353 - copyprop iteration order

After recent improvements to copyprop to catch more constants
it shows that the current iteration order prefering forward
progress over iterating doesn't make much sense for an SSA
propagator.  The following instead first iterates cycles which
makes sure to not start with optimistically constant PHIs out
of cycles that optimistically do not exit.

	PR tree-optimization/108353
	* tree-ssa-propagate.cc (cfg_blocks_back, ssa_edge_worklist_back):
	Remove.
	(add_ssa_edge): Simplify.
	(add_control_edge): Likewise.
	(ssa_prop_init): Likewise.
	(ssa_prop_fini): Likewise.
	(ssa_propagation_engine::ssa_propagate): Likewise.

	* gcc.dg/tree-ssa/ssa-copyprop-3.c: New testcase.