cmd/gc: don't walk static nodes generated by anylit.

During anylit run, nodes such as SLICEARR(statictmp, [:])
may be generated and are expected to be found unchanged by
gen_as_init.

In some walks (in particular walkselect), the statement
may be walked again and lowered to its usual form, leading to a
crash.

Fixes #8017.
Fixes #8024.
Fixes #8058.

LGTM=rsc
R=golang-codereviews, dvyukov, gobot, rsc
CC=golang-codereviews
https://golang.org/cl/112080043

src/cmd/gc/walk.c

@@ -137,6 +137,8 @@ walkstmt(Node **np)
 	n = *np;
 	if(n == N)
 		return;
+	if(n->dodata == 2) // don't walk, generated by anylit.
+		return;
 
 	setlineno(n);
 

test/fixedbugs/issue8017.go

+// compile
+
+// Copyright 2014 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Issues 8017 and 8058: walk modifies nodes generated
+// by slicelit and causes an internal error afterwards
+// when gen_as_init parses it back.
+
+package main
+
+func F() {
+	var ch chan int
+	select {
+	case <-ch:
+	case <-make(chan int, len([2][]int{([][]int{})[len(ch)], []int{}})):
+	}
+}
+
+func G() {
+	select {
+	case <-([1][]chan int{[]chan int{}})[0][0]:
+	default:
+	}
+}