Turn on Sk4px xfermodes when we have NEON too.

src/opts/SkNx_neon.h

Index: src/opts/SkNx_neon.h
diff --git a/src/opts/SkNx_neon.h b/src/opts/SkNx_neon.h
index b9d4357e5a306f204587e83259036dd857232836..e4dbec90830a294994023dd5784f6337c3f08539 100644
--- a/src/opts/SkNx_neon.h
+++ b/src/opts/SkNx_neon.h
@@ -355,6 +355,8 @@ public:
 
     void store(uint8_t vals[16]) const { vst1q_u8(vals, fVec); }
 
+    SkNi saturatedAdd(const SkNi& o) const { return vqaddq_u8(fVec, o.fVec); }
+
     SkNi operator + (const SkNi& o) const { return vaddq_u8(fVec, o.fVec); }
     SkNi operator - (const SkNi& o) const { return vsubq_u8(fVec, o.fVec); }
     SkNi operator * (const SkNi& o) const { return vmulq_u8(fVec, o.fVec); }