Fix and speedup IDF pruning by dominator

Fix and speedup IDF pruning by dominator

When insert_updated_phi_nodes_for tries to skip pruning the IDF to
blocks dominated by the nearest common dominator of the set of
definition blocks it compares against ENTRY_BLOCK but that's never
going to be the common dominator.  In fact if it ever were the code
fails to copy IDF to PRUNED_IDF, leading to wrong code.

The following fixes that by avoiding the copy and pruning from the
IDF in-place as well as using the more approprate check against
the single successor of the ENTRY_BLOCK.

	* tree-into-ssa.cc (insert_updated_phi_nodes_for): Skip
	pruning when the nearest common dominator is the successor
	of ENTRY_BLOCK.  Do not copy IDF but prune it directly.