Use RPO order for sinking

The following makes us use RPO order instead of walking post-dominators.
This ensures we visit a block before any predecessors.  I've seen
some extra sinking because of this in a larger testcase but failed
to reduce a smaller one (processing of post-dominator sons is unordered
so I failed to have "luck").

	* tree-ssa-sink.cc (pass_sink_code::execute): Do not
	calculate post-dominators.  Calculate RPO on the inverted
	graph and process blocks in that order.