Remove duplicate DFS walks from DF init

The following removes unused CFG order computes from
rest_of_handle_df_initialize.  The CFG orders are computed from df_analyze ().
This also removes code duplication that would have to be kept in sync.

	* df-core.cc (rest_of_handle_df_initialize): Remove
	computation of df->postorder, df->postorder_inverted and
	df->n_blocks.