gc: fix error for +string

Fixes #1710.

R=ken2
CC=golang-dev
https://golang.org/cl/4444054

src/cmd/gc/align.c

@@ -530,7 +530,7 @@ typeinit(void)
 	okfor[OCOM] = okforand;
 	okfor[OMINUS] = okforarith;
 	okfor[ONOT] = okforbool;
-	okfor[OPLUS] = okforadd;
+	okfor[OPLUS] = okforarith;
 
 	// special
 	okfor[OCAP] = okforcap;

test/fixedbugs/bug330.go

@@ -0,0 +1,13 @@
+// errchk $G -e $D/$F.go
+
+// Copyright 2011 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
+
+func main() {
+	x := ""
+	x = +"hello"  // ERROR "invalid operation.*string"
+	x = +x  // ERROR "invalid operation.*string"
+}