Ensure we do not clobber the register holding the elements backing store.

test/mjsunit/regress/regress-crbug-162085.js

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // 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.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 12 matching lines @@
 // DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 // THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 // Ensure extending an empty packed smi array with a double initializes the
 // array with holes.
 var a = [1,2,3];
 a.length = 0;
 a[0] = 1.4;
+assertEquals(1.4, a[0]);
 assertEquals(undefined, a[1]);
 assertEquals(undefined, a[2]);
 assertEquals(undefined, a[3]);
 
 // Ensure the double array growstub initializes the array with holes.
 function grow_store(a,i,v) {
   a[i] = v;
 }
 
 var a2 = [1.3];
 grow_store(a2,1,1.4);
 a2.length = 0;
 grow_store(a2,0,1.5);
+assertEquals(1.5, a2[0]);
 assertEquals(undefined, a2[1]);
 assertEquals(undefined, a2[2]);
 assertEquals(undefined, a2[3]);
 
 // Check storing objects using the double grow stub.
 var a3 = [1.3];
 var o = {};
 grow_store(a3, 1, o);
 assertEquals(1.3, a3[0]);
 assertEquals(o, a3[1]);
+
+// Ensure the double array growstub initializes the array with holes.
+function grow_store2(a,i,v) {
+  a[i] = v;
+}
+
+var a4 = [1.3];
+grow_store2(a4,1,1.4);
+a4.length = 0;
+grow_store2(a4,0,1);
+assertEquals(1, a4[0]);
+assertEquals(undefined, a4[1]);
+assertEquals(undefined, a4[2]);
+assertEquals(undefined, a4[3]);