constant conversion of int (non-ideal) constant doesn't work

R=rsc
DELTA=20  (20 added, 0 deleted, 0 changed)
OCL=26054
CL=26062

test/bugs/bug138.go

+// Copyright 2009 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.
+
+// $G $D/$F.go || echo BUG should compile
+
+package main
+
+func main() {
+	const c int = -1;
+	var i int = -1;
+	var xc uint = uint(c);  // this does not work
+	var xi uint = uint(i);  // this works
+}
+
+/*
+bug138.go:8: overflow converting constant to uint
+bug138.go:8: illegal combination of literals CONV 7
+*/

test/golden.out

@@ -154,6 +154,11 @@ bugs/bug137.go:9: break label is not defined: L2
 bugs/bug137.go:15: break label is not defined: L4
 BUG should compile
 
+=========== bugs/bug138.go
+bugs/bug138.go:8: overflow converting constant to uint
+bugs/bug138.go:8: illegal combination of literals CONV 7
+BUG should compile
+
 =========== fixedbugs/bug016.go
 fixedbugs/bug016.go:7: overflow converting constant to uint