Fix JSArray-specific length lookup in polymorphic array handling

test/mjsunit/regress/regress-crbug-263276.js

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 // Flags: --allow-natives-syntax
 
-function TestNonSmiArrayLength() {
-  function f(a) {
-    return a.length+1;
-  }
+var array1 = [];
+array1.foo = true;
 
-  var a = [];
-  a.length = 0xFFFF;
-  assertSame(0x10000, f(a));
-  assertSame(0x10000, f(a));
+var array2 = [];
+array2.bar = true;
 
-  %OptimizeFunctionOnNextCall(f);
-  a.length = 0xFFFFFFFF;
-  assertSame(0x100000000, f(a));
+function bad(array) {
+  array[array.length] = 1;
 }
 
-TestNonSmiArrayLength();
-
+bad(array1);
+bad(array1);
+bad(array2);  // Length is now 1.
+bad(array2);  // Length is now 2.
+%OptimizeFunctionOnNextCall(bad);
+bad(array2);  // Length is now 3.
+assertEquals(3, array2.length);