[arm] Recognize SXTB, SXTH, UXTB and UXTH.

test/mjsunit/asm/sign-extend.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.
+
+var stdlib = this;
+var buffer = new ArrayBuffer(64 * 1024);
+var foreign = {}
+
+
+var sext8 = (function Module(stdlib, foreign, heap) {
+  "use asm";
+  function sext8(i) {
+    i = i|0;
+    i = i << 24 >> 24;
+    return i|0;
+  }
+  return { sext8: sext8 };
+})(stdlib, foreign, buffer).sext8;
+
+assertEquals(-128, sext8(128));
+assertEquals(-1, sext8(-1));
+assertEquals(-1, sext8(255));
+assertEquals(0, sext8(0));
+assertEquals(0, sext8(256));
+assertEquals(42, sext8(42));
+assertEquals(127, sext8(127));
+
+
+var sext16 = (function Module(stdlib, foreign, heap) {
+  "use asm";
+  function sext16(i) {
+    i = i|0;
+    i = i << 16 >> 16;
+    return i|0;
+  }
+  return { sext16: sext16 };
+})(stdlib, foreign, buffer).sext16;
+
+assertEquals(-32768, sext16(32768));
+assertEquals(-1, sext16(-1));
+assertEquals(-1, sext16(65535));
+assertEquals(0, sext16(0));
+assertEquals(0, sext16(65536));
+assertEquals(128, sext16(128));
+assertEquals(32767, sext16(32767));