cmd/compile: add test for OPmodify ops clobbering flags

Code fix was in CL 122556.  This is a corresponding test case.

Fixes #26426

Change-Id: Ib8769f367aed8bead029da0a8d2ddccee1d1dccb
Reviewed-on: https://go-review.googlesource.com/124535
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

test/fixedbugs/issue26426.go

+//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.
+
+package main
+
+//go:noinline
+func f(p *int, v int, q1, q2 *int, r *bool) {
+	x := *r
+	if x {
+		*q1 = 1
+	}
+	*p = *p + v // This must clobber flags. Otherwise we keep x in a flags register.
+	if x {
+		*q2 = 1
+	}
+}
+
+func main() {
+	var p int
+	var q1, q2 int
+	var b bool
+	f(&p, 1, &q1, &q2, &b)
+	if q1 != 0 || q2 != 0 {
+		panic("bad")
+	}
+}