cmd/compile: prevent ICE from misuse of [...]T arrays

Fixes #16428.

Change-Id: I78d37472e228402bb3c06d7ebd441952386fa38a
Reviewed-on: https://go-review.googlesource.com/31731
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

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

@@ -344,12 +344,15 @@ OpSwitch:
 		if n.Left == nil {
 			t = typSlice(r.Type)
 		} else if n.Left.Op == ODDD {
-			t = typDDDArray(r.Type)
-			if top&Ecomplit == 0 && n.Diag == 0 {
-				t.Broke = true
+			if top&Ecomplit == 0 {
+				if n.Diag == 0 {
 					n.Diag = 1
 					yyerror("use of [...] array outside of array literal")
 				}
+				n.Type = nil
+				return n
+			}
+			t = typDDDArray(r.Type)
 		} else {
 			n.Left = indexlit(typecheck(n.Left, Erv))
 			l := n.Left

test/fixedbugs/issue16428.go

+// errorcheck
+
+// Copyright 2016 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.
+
+package p
+
+var (
+	b = [...]byte("abc") // ERROR "outside of array literal"
+	s = len(b)
+)