cmd/compile: add missing OASOP case in mayModifyPkgVar

CL 395541 made staticopy safe, stop applying the optimization once
seeing an expression that may modify global variables. However, it
misses the case for OASOP expression, causing the static init
mis-recognizes the modification and think it's safe.

Fixing this by adding missing OASOP case.

Fixes #66585

Change-Id: I603cec018d3b5a09825c14e1f066a0e16f8bde23
Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest
Reviewed-on: https://go-review.googlesource.com/c/go/+/575216
Reviewed-by: Keith Randall <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
Reviewed-by: Than McIntosh <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: cuonglm

src/cmd/compile/internal/staticinit/sched.go

@@ -899,6 +899,12 @@ func mayModifyPkgVar(n ir.Node) bool {
 		case ir.OAPPEND, ir.OCLEAR, ir.OCOPY:
 			return true // could mutate a global array
 
+		case ir.OASOP:
+			n := n.(*ir.AssignOpStmt)
+			if !safeLHS(n.X) {
+				return true
+			}
+
 		case ir.OAS:
 			n := n.(*ir.AssignStmt)
 			if !safeLHS(n.X) {

test/fixedbugs/issue66585.go

@@ -0,0 +1,25 @@
+// run
+
+// Copyright 2024 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 x = 0
+var a = foo()
+var b = x
+
+func foo() int {
+	x++
+	return x
+}
+
+func main() {
+	if a != 1 {
+		panic("unexpected a value")
+	}
+	if b != 1 {
+		panic("unexpected b value")
+	}
+}