Add GSI_LAST_NEW_STMT iterator update

Add GSI_LAST_NEW_STMT iterator update

Currently when adding a sequence before there's no way to get the
iterator placed at the last added stmt which results in convoluted
code in the if-conversion usecase.  The following adds
GSI_LAST_NEW_STMT and corrects one obvious mistake in
execute_update_addresses_taken as well as tries to avoid the
just filed PR102726 by biasing the enum values to be outside of
the boolean 0/1 range.

2021-10-13  Richard Biener  <rguenther@suse.de>

	* gimple-iterator.h (gsi_iterator_update): Add GSI_LAST_NEW_STMT,
	start at integer value 2.
	* gimple-iterator.c (gsi_insert_seq_nodes_before): Update
	the iterator for GSI_LAST_NEW_STMT.
	(gsi_insert_seq_nodes_after): Likewise.
	* tree-if-conv.c (predicate_statements): Use GSI_LAST_NEW_STMT.
	* tree-ssa.c (execute_update_addresses_taken): Correct bogus
	arguments to gsi_replace.