Refactors CPU feature detection.

runtime/vm/assembler_x64.h

Index: runtime/vm/assembler_x64.h
===================================================================
--- runtime/vm/assembler_x64.h	(revision 32467)
+++ runtime/vm/assembler_x64.h	(working copy)
@@ -317,24 +317,6 @@
 };
 
 
-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);