[asm.js] Fix typing bug for non-literals in heap access.

test/mjsunit/regress/regress-599825.js

+// Copyright 2016 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: --expose-wasm
+
+function __f_97(stdlib, buffer) {
+  "use asm";
+  var __v_30 = new stdlib.Int32Array(buffer);
+  function __f_74() {
+    var __v_27 = 4;
+    __v_30[__v_27 >> __v_2] = ((__v_30[-1073741825]|-10) + 2) | 0;
+  }
+}
+assertThrows(function() {
+  var module = Wasm.instantiateModuleFromAsm( __f_97.toString());
+});