cmd/compile: change Node fields from *NodeList to Nodes

Compile time is about the same.  Getting rid of the nodeSeq interfaces,
particularly nodeSeqIterate, should produce some improvements.

Passes toolstash -cmp.

Update #14473.

Change-Id: I678abafdd9129c6cccb0ec980511932eaed496a0
Reviewed-on: https://go-review.googlesource.com/20343
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -15,10 +15,10 @@ type Node struct {
 	// Generic recursive walks should follow these fields.
 	Left  *Node
 	Right *Node
-	Ninit *NodeList
+	Ninit Nodes
 	Nbody Nodes
-	List  *NodeList
-	Rlist *NodeList
+	List  Nodes
+	Rlist Nodes
 
 	// most nodes
 	Type *Type