go/types: fix regression in type checking of RangeStmt.

Now that untyped expressions are done in two phases, the
identity of operand.expr is used as a map key; when reusing
operand values we now must be careful to update the
expr field.

R=gri
CC=golang-dev
https://golang.org/cl/7444049

src/pkg/go/types/stmt.go

@@ -719,6 +719,7 @@ func (check *checker) stmt(s ast.Stmt) {
 		x.mode = value
 		if s.Key != nil {
 			x.typ = key
+			x.expr = s.Key
 			check.assign1to1(s.Key, nil, &x, decl, -1)
 		} else {
 			check.invalidAST(s.Pos(), "range clause requires index iteration variable")
@@ -726,6 +727,7 @@ func (check *checker) stmt(s ast.Stmt) {
 		}
 		if s.Value != nil {
 			x.typ = val
+			x.expr = s.Value
 			check.assign1to1(s.Value, nil, &x, decl, -1)
 		}