middle-end/108500 - replace recursive domtree DFS traversal

middle-end/108500 - replace recursive domtree DFS traversal

The following replaces the recursive DFS traversal of the dominator
tree in assign_dfs_numbers with a tree traversal using the fact
that we have recorded parents.

Bootstrapped and tested on x86_64-unknown-linux-gnu.

This makes r13-5325 somewhat obsolete, though not computing the
DFS numbers at all is beneficial in the cases where we perform
immediate CFG manipulations.

OK for trunk and later branch(es)?

Thanks,
Richard.

	PR middle-end/108500
	* dominance.cc (assign_dfs_numbers): Replace recursive DFS
	with tree traversal algorithm.