cmd/compile: be explicit about NodeList List storage stealing

This only matters in a couple of places.
Be explicit about it.

This helps to narrow the scope of any given
NodeList-related change.

Change-Id: I506f2ee0417bc6eb70d4bc0456d87a92cd6f7973

src/cmd/compile/internal/gc/go.y

@@ -774,7 +774,7 @@ if_stmt:
 			if nn.N.Op == OIF {
 				popdcl();
 			}
-			n.Rlist = list1(nn.N);
+			n.Rlist = list1Steal(nn.N)
 			n = nn.N;
 		}
 	}
@@ -1811,7 +1811,7 @@ non_dcl_stmt:
 		$1.Name.Defn = $4;
 		l = list1($1);
 		if $4 != nil {
-			l = list(l, $4);
+			l = listSteal(l, $4)
 		}
 		$$ = liststmt(l);
 	}
@@ -1868,14 +1868,14 @@ stmt_list:
 	{
 		$$ = nil;
 		if $1 != nil {
-			$$ = list1($1);
+			$$ = list1Steal($1)
 		}
 	}
 |	stmt_list ';' stmt
 	{
 		$$ = $1;
 		if $3 != nil {
-			$$ = list($$, $3);
+			$$ = listSteal($$, $3)
 		}
 	}
 

src/cmd/compile/internal/gc/syntax.go

@@ -387,15 +387,6 @@ func list1(n *Node) *NodeList {
 	if n == nil {
 		return nil
 	}
-	if n.Op == OBLOCK && n.Ninit == nil {
-		// Flatten list and steal storage.
-		// Poison pointer to catch errant uses.
-		l := n.List
-
-		n.List = nil
-		return l
-	}
-
 	l := new(NodeList)
 	l.N = n
 	l.End = l
@@ -407,6 +398,24 @@ func list(l *NodeList, n *Node) *NodeList {
 	return concat(l, list1(n))
 }
 
+func list1Steal(n *Node) *NodeList {
+	if n == nil {
+		return nil
+	}
+	if n.Op == OBLOCK && n.Ninit == nil {
+		// Flatten list and steal storage.
+		// Poison pointer to catch errant uses.
+		l := n.List
+		n.List = nil
+		return l
+	}
+	return list1(n)
+}
+
+func listSteal(l *NodeList, n *Node) *NodeList {
+	return concat(l, list1Steal(n))
+}
+
 // listsort sorts *l in place according to the 3-way comparison function f.
 // The algorithm is mergesort, so it is guaranteed to be O(n log n).
 func listsort(l **NodeList, f func(*Node, *Node) int) {

src/cmd/compile/internal/gc/y.go

@@ -1882,7 +1882,7 @@ yydefault:
 				if nn.N.Op == OIF {
 					popdcl()
 				}
-				n.Rlist = list1(nn.N)
+				n.Rlist = list1Steal(nn.N)
 				n = nn.N
 			}
 		}
@@ -2985,7 +2985,7 @@ yydefault:
 			yyDollar[1].node.Name.Defn = yyDollar[4].node
 			l = list1(yyDollar[1].node)
 			if yyDollar[4].node != nil {
-				l = list(l, yyDollar[4].node)
+				l = listSteal(l, yyDollar[4].node)
 			}
 			yyVAL.node = liststmt(l)
 		}
@@ -3056,7 +3056,7 @@ yydefault:
 		{
 			yyVAL.list = nil
 			if yyDollar[1].node != nil {
-				yyVAL.list = list1(yyDollar[1].node)
+				yyVAL.list = list1Steal(yyDollar[1].node)
 			}
 		}
 	case 271:
@@ -3065,7 +3065,7 @@ yydefault:
 		{
 			yyVAL.list = yyDollar[1].list
 			if yyDollar[3].node != nil {
-				yyVAL.list = list(yyVAL.list, yyDollar[3].node)
+				yyVAL.list = listSteal(yyVAL.list, yyDollar[3].node)
 			}
 		}
 	case 272: