test: add order of evaluation test case that gccgo got wrong

Updates #23188

Change-Id: Idc5567546d1c4c592f997a4cebbbf483b85331e0
Reviewed-on: https://go-review.googlesource.com/123115
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

test/fixedbugs/issue23188.go

@@ -0,0 +1,32 @@
+// run
+
+// Copyright 2018 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.
+
+// Test order of evaluation of index operations.
+
+package main
+
+func main() {
+	arr := []int{1, 2}
+
+	// The spec says that in an assignment statement the operands
+	// of all index expressions and pointer indirections on the
+	// left, and the expressions on the right, are evaluated in
+	// the usual order. The usual order means function calls and
+	// channel operations are done first. Then the assignments are
+	// carried out one at a time. The operands of an index
+	// expression include both the array and the index. So this
+	// evaluates as
+	//   tmp1 := arr
+	//   tmp2 := len(arr) - 1
+	//   tmp3 := len(arr)
+	//   arr = arr[:tmp3-1]
+	//   tmp1[tmp2] = 3
+	arr, arr[len(arr)-1] = arr[:len(arr)-1], 3
+
+	if len(arr) != 1 || arr[0] != 1 || arr[:2][1] != 3 {
+		panic(arr)
+	}
+}