fhdl/tools: BUGFIX: fix group_by_target grouping

group_by_target does not properly combine target groups if statements
are presented in the order:

 ({A}, statement1)
 ({B}, statement2)
 ({A, B}, statement3)

which returns groups:

 ({A, B}, [statement1, statement3])
 ({B}, [statement2])

This patch fixes group_by_target such that the resulting group is:

 ({A, B}, [statement1, statement2, statement3])

migen/fhdl/tools.py

@@ -48,20 +48,29 @@ def list_targets(node):
 	lister.visit(node)
 	return lister.output_list
 
+def resort_statements(ol):
+	return [statement for i, statement in
+			sorted(ol, key=lambda x: x[0])]
+
 def group_by_targets(sl):
 	groups = []
-	for statement in flat_iteration(sl):
+	for statement_order, statement in enumerate(flat_iteration(sl)):
 		targets = list_targets(statement)
-		processed = False
-		for g in groups:
-			if not targets.isdisjoint(g[0]):
-				g[0].update(targets)
-				g[1].append(statement)
-				processed = True
-				break
-		if not processed:
-			groups.append((targets, [statement]))
-	return groups
+
+		chk_groups = [(targets.isdisjoint(g[0]), g) for g in groups]
+		merge_groups = [g for dj, g in chk_groups if not dj]
+		groups = [g for dj, g in chk_groups if dj]
+
+		new_group = (set(targets), [(statement_order, statement)])
+
+		for g in merge_groups:
+			new_group[0].update(g[0])
+			new_group[1].extend(g[1])
+
+		groups.append(new_group)
+
+	return [(target, resort_statements(stmts))
+		for target, stmts in groups]
 
 def list_special_ios(f, ins, outs, inouts):
 	r = set()