re PR tree-optimization/42172 (inefficient bit fields assignments)

	PR target/42172
	* combine.c (combine_validate_cost): New arg I0.  All callers changed.
	Take its cost into account if nonnull.
	(insn_a_feeds_b): New static function.
	(combine_instructions): Look for four-insn combinations.
	(can_combine_p): New args PRED2, SUCC2.  All callers changed.  Take
	them into account when computing all_adjacent and looking for other
	uses.
	(combinable_i3pat): New args I0DEST, I0_NOT_IN_SRC.  All callers
	changed.  Treat them like I1DEST and I1_NOT_IN_SRC.
	(try_combine): New arg I0.  Handle four-insn combinations.
	(distribute_notes): New arg ELIM_I0.  All callers changed.  Treat it
	like ELIM_I1.

From-SVN: r163383