cmd/compile: improve single blank variable handling in walkrange

Refactor walkrange to treat "for _ = range a" as "for range a".

This avoids generating some later discarded nodes in the compiler.

Passes toolstash -cmp.

Change-Id: Ifb2e1ca3b8519cbb67e8ad5aad514af9d18f1ec4
Reviewed-on: https://go-review.googlesource.com/61017
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -155,27 +155,36 @@ func walkrange(n *Node) *Node {
 	lno := setlineno(a)
 	n.Right = nil
 
-	var v1 *Node
-	if n.List.Len() != 0 {
+	var v1, v2 *Node
+	l := n.List.Len()
+	if l > 0 {
 		v1 = n.List.First()
 	}
-	var v2 *Node
-	if n.List.Len() > 1 && !isblank(n.List.Second()) {
+
+	if l > 1 {
 		v2 = n.List.Second()
 	}
 
+	if isblank(v2) {
+		v2 = nil
+	}
+
+	if isblank(v1) && v2 == nil {
+		v1 = nil
+	}
+
 	if v1 == nil && v2 != nil {
 		Fatalf("walkrange: v2 != nil while v1 == nil")
 	}
 
-	var ifGuard *Node
-
-	translatedLoopOp := OFOR
-
 	// n.List has no meaning anymore, clear it
 	// to avoid erroneous processing by racewalk.
 	n.List.Set(nil)
 
+	var ifGuard *Node
+
+	translatedLoopOp := OFOR
+
 	var body []*Node
 	var init []*Node
 	switch t.Etype {