Tag stubs that rely on instance types as MEGAMORPHIC.

test/mjsunit/regress/regress-crbug-173974.js

Index: test/mjsunit/regress/regress-crbug-173974.js
diff --git a/test/mjsunit/regress/regress-crbug-172345.js b/test/mjsunit/regress/regress-crbug-173974.js
similarity index 92%
copy from test/mjsunit/regress/regress-crbug-172345.js
copy to test/mjsunit/regress/regress-crbug-173974.js
index 711501caa791f2974635ac4dd894568a2e3fc982..905bd6058a0ad0fe2ebe10e4c7dafbe9945cbe3b 100644
--- a/test/mjsunit/regress/regress-crbug-172345.js
+++ b/test/mjsunit/regress/regress-crbug-173974.js
@@ -25,10 +25,12 @@
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-function f(a,i) {
-  return a[i];
-}
+// Flags: --allow-natives-syntax
 
-f([1,2,3], "length");
-f([1,2,3], "length");
-f([1,2,3], 2);
+function f() {
+  var count = "";
+  count[0] --;
+}
+f();
+%OptimizeFunctionOnNextCall(f);
+f();