gimple-iterator: Some gsi_safe_insert_*before fixes

gimple-iterator: Some gsi_safe_insert_*before fixes

When trying to use the gsi_safe_insert*before APIs in bitint lowering,
I've discovered 3 issues and the following patch addresses those:

1) both split_block and split_edge update CDI_DOMINATORS if they are
   available, but because edge_before_returns_twice_call first splits
   and then adds an extra EDGE_ABNORMAL edge and then removes another
   one, the immediate dominators of both the new bb and the bb with
   returns_twice call need to change
2) the new EDGE_ABNORMAL edge had uninitialized probability; this patch
   copies the probability from the edge that is going to be removed
   and similarly copies other flags (EDGE_EXECUTABLE, EDGE_DFS_BACK,
   EDGE_IRREDUCIBLE_LOOP etc.)
3) if edge_before_returns_twice_call splits a block, then the bb with
   returns_twice call changes, so the gimple_stmt_iterator for it is
   no longer accurate, it points to the right statement, but gsi_bb
   and gsi_seq are no longer correct; the patch updates it

2024-03-14  Jakub Jelinek  <jakub@redhat.com>

	* gimple-iterator.cc (edge_before_returns_twice_call): Copy all
	flags and probability from ad_edge to e edge.  If CDI_DOMINATORS
	are computed, recompute immediate dominator of other_edge->src
	and other_edge->dest.
	(gsi_safe_insert_before, gsi_safe_insert_seq_before): Update *iter
	for the returns_twice call case to the gsi_for_stmt (stmt) to deal
	with update it for bb splitting.