Simplify flattening to just write enough to call the factory

src/opts/SkXfermode_opts_SSE2.cpp

Index: src/opts/SkXfermode_opts_SSE2.cpp
diff --git a/src/opts/SkXfermode_opts_SSE2.cpp b/src/opts/SkXfermode_opts_SSE2.cpp
index 94f9a4aea3be1591323f603b3d3a8d88fe7ea79a..02ca0382dd8b91f7b843107073ffdf65038fa30e 100644
--- a/src/opts/SkXfermode_opts_SSE2.cpp
+++ b/src/opts/SkXfermode_opts_SSE2.cpp
@@ -641,11 +641,12 @@ typedef __m128i (*SkXfermodeProcSIMD)(const __m128i& src, const __m128i& dst);
 
 extern SkXfermodeProcSIMD gSSE2XfermodeProcs[];
 
-SkSSE2ProcCoeffXfermode::SkSSE2ProcCoeffXfermode(SkReadBuffer& buffer)
-    : INHERITED(buffer) {
+#ifdef SK_SUPPORT_LEGACY_DEEPFLATTENING
+SkSSE2ProcCoeffXfermode::SkSSE2ProcCoeffXfermode(SkReadBuffer& buffer) : INHERITED(buffer) {
     fProcSIMD = reinterpret_cast<void*>(gSSE2XfermodeProcs[this->getMode()]);
     buffer.validate(fProcSIMD != NULL);
 }
+#endif
 
 void SkSSE2ProcCoeffXfermode::xfer32(SkPMColor dst[], const SkPMColor src[],
                                      int count, const SkAlpha aa[]) const {