gc: don't inherit orig from subnodes in constant expression nodes.

The wrong value made Nconv() show "1" for node "-1", and "2" from
node "2+3".
Fixes #2452.

R=gri, lvd, rsc
CC=golang-dev, remy
https://golang.org/cl/5435064

src/cmd/gc/const.c

@@ -424,7 +424,7 @@ isconst(Node *n, int ct)
 void
 evconst(Node *n)
 {
-	Node *nl, *nr;
+	Node *nl, *nr, *norig;
 	int32 len;
 	Strlit *str;
 	int wl, wr, lno, et;
@@ -842,8 +842,11 @@ unary:
 	}
 
 ret:
+	norig = n->orig;
 	// rewrite n in place.
 	*n = *nl;
+	// restore value of n->orig.
+	n->orig = norig;
 	n->val = v;
 
 	// check range.

test/fixedbugs/bug379.go

@@ -0,0 +1,18 @@
+// errchk $G $D/$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.
+
+// Issue 2452.
+
+// Check that the error messages says 
+//	bug378.go:17: 3 not used
+// and not
+//	bug378.go:17: 1 not used
+
+package main
+
+func main() {
+	1 + 2 // ERROR "3 not used|value computed is not used"
+}