Reverts refactoring of CPU feature detection for

runtime/vm/assembler_x64.h

Index: runtime/vm/assembler_x64.h
===================================================================
--- runtime/vm/assembler_x64.h	(revision 32559)
+++ runtime/vm/assembler_x64.h	(working copy)
@@ -317,6 +317,24 @@
 };
 
 
+class CPUFeatures : public AllStatic {
+ public:
+  static void InitOnce();
+  // x64 always has at least SSE2.
+  static bool sse2_supported() { return true; }
+  static bool sse4_1_supported();
+  static bool double_truncate_round_supported() { return sse4_1_supported(); }
+
+ private:
+  static const uint64_t kSSE4_1BitMask = static_cast<uint64_t>(1) << 51;
+
+  static bool sse4_1_supported_;
+#ifdef DEBUG
+  static bool initialized_;
+#endif
+};
+
+
 class Assembler : public ValueObject {
  public:
   explicit Assembler(bool use_far_branches = false);