Fix setting array length to zero for slow elements.

test/mjsunit/regress/regress-crbug-146910.js

 // Copyright 2012 the V8 project authors. All rights reserved.
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 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
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-// Flags: --allow-natives-syntax
+var x = [];
+assertSame(0, x.length);
+assertSame(undefined, x[0]);
 
-// Verify that LRandom preserves rsi correctly.
+Object.defineProperty(x, '0', { value: 7, configurable: false });
+assertSame(1, x.length);
+assertSame(7, x[0]);
 
-function foo() {
-  Math.random();
-  new Function("");
-}
-
-foo();
-foo();
-foo();
-%OptimizeFunctionOnNextCall(foo);
-foo();
+x.length = 0;
+assertSame(1, x.length);
+assertSame(7, x[0]);