test/fixedbugs: add test for divide by zero being optimized away

This only tests amd64 because it's currently broken on non-SSA
backends.

Fixes #8613

Change-Id: I6bc501c81c395e533bb9c7335789750e0c6b7a8f
Reviewed-on: https://go-review.googlesource.com/21325Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

test/fixedbugs/issue8613.go

+// +build amd64
+// run
+
+// 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 main
+
+var out int
+var zero int
+
+func main() {
+	wantPanic("test1", func() {
+		out = 1 / zero
+	})
+	wantPanic("test2", func() {
+		_ = 1 / zero
+	})
+	wantPanic("test3", func() {
+		v := 0
+		_ = 1 / v
+	})
+	wantPanic("test4", func() { divby(0) })
+}
+
+func wantPanic(test string, fn func()) {
+	defer func() {
+		if e := recover(); e == nil {
+			panic(test + ": expected panic")
+		}
+	}()
+	fn()
+}
+
+//go:noinline
+func divby(v int) {
+	_ = 1 / v
+}