Initialize CompareOperation::combined_type to avoid crashes.

BUG=
R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/23811003

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@16512 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/ast.h

@@ -2038,7 +2038,8 @@ class CompareOperation V8_FINAL : public Expression {
         op_(op),
         left_(left),
         right_(right),
-        pos_(pos) {
+        pos_(pos),
+        combined_type_(Type::Null(), isolate) {
     ASSERT(Token::IsCompareOp(op));
   }