Reorg Sk4x to match the pattern of SkPMFloat.

src/core/Sk4x.h

 #ifndef Sk4x_DEFINED
 #define Sk4x_DEFINED
 
 #include "SkTypes.h"
 
 #define SK4X_PREAMBLE 1
     #if SK_CPU_SSE_LEVEL >= SK_CPU_SSE_LEVEL_SSE2
-        #include "Sk4x_sse.h"
+        #include "../opts/Sk4x_sse.h"
     #elif defined(SK_ARM_HAS_NEON)
-        #include "Sk4x_neon.h"
+        #include "../opts/Sk4x_neon.h"
     #else
-        #include "Sk4x_portable.h"
+        #include "../opts/Sk4x_none.h"
     #endif
 #undef SK4X_PREAMBLE
 
 template <typename T> class Sk4x;
 typedef Sk4x<float>   Sk4f;
 typedef Sk4x<int32_t> Sk4i;
 
 // Some Sk4x methods are implemented only for Sk4f or Sk4i.
 // They might be unavailable, really slow, or just a bad idea.
 // Talk to mtklein if you find yourself unable to link and
@@ skipping 64 matching lines @@
     // TODO: these are particularly efficient in SSE.  Useful?  Also efficient in NEON?
     // static Sk4x XAYB(const Sk4x& xyzw, const Sk4x& abcd);
     // static Sk4x ZCWD(const Sk4x& xyzw, const Sk4x& abcd);
 
 private:
     // It's handy to have Sk4f and Sk4i be mutual friends.
     template <typename S> friend class Sk4x;
 
 #define SK4X_PRIVATE 1
     #if SK_CPU_SSE_LEVEL >= SK_CPU_SSE_LEVEL_SSE2
-        #include "Sk4x_sse.h"
+        #include "../opts/Sk4x_sse.h"
     #elif defined(SK_ARM_HAS_NEON)
-        #include "Sk4x_neon.h"
+        #include "../opts/Sk4x_neon.h"
     #else
-        #include "Sk4x_portable.h"
+        #include "../opts/Sk4x_none.h"
     #endif
 #undef SK4X_PRIVATE
 };
 
 #if SK_CPU_SSE_LEVEL >= SK_CPU_SSE_LEVEL_SSE2
-    #include "Sk4x_sse.h"
+    #include "../opts/Sk4x_sse.h"
 #elif defined(SK_ARM_HAS_NEON)
-    #include "Sk4x_neon.h"
+    #include "../opts/Sk4x_neon.h"
 #else
-    #include "Sk4x_portable.h"
+    #include "../opts/Sk4x_none.h"
 #endif
 
 #endif//Sk4x_DEFINED