Fix index register assignment in LoadFieldByIndex for arm, arm64, and mips.

test/mjsunit/regress/regress-368243.js

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Flags: --allow-natives-syntax
 
-function f() {};
-int_array = [1];
+function foo(a, c){
+  for(var f in c) {
+    if ("object" === typeof c[f]) {
+      a[f] = c[f];
+      foo(a[f], c[f]);
+    }
+  }
+};
 
-function foo() {
-  var x;
-  for (var i = -1; i < 0; i++) {
-    x = int_array[i + 1];
-    f(function() { x = i; });
-  }
-}
+c = {
+  "one" : { x : 1},
+  "two" : { x : 2},
+  "thr" : { x : 3, z : 4},
+};
 
-foo();
+foo({}, c);
+foo({}, c);
 %OptimizeFunctionOnNextCall(foo);
-foo();
+foo({}, c);