Add swizzle for rgb8888.

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 1acd7a00f40ffc6df9591dc5b923157d4c64e3a8..52b2e73798e7eebb69144fb75288eba151553a23 100644
--- a/src/opts/SkNx_neon.h
+++ b/src/opts/SkNx_neon.h
@@ -305,12 +305,20 @@ public:
     SkNx(const uint8x8_t& vec) : fVec(vec) {}
 
     SkNx() {}
+    SkNx(uint8_t a, uint8_t b, uint8_t c, uint8_t d) {
+        fVec = (uint8x8_t){a,b,c,d, 0,0,0,0};
+    }
     static SkNx Load(const void* ptr) {
         return (uint8x8_t)vld1_dup_u32((const uint32_t*)ptr);
     }
     void store(void* ptr) const {
         return vst1_lane_u32((uint32_t*)ptr, (uint32x2_t)fVec, 0);
     }
+    uint8_t operator[](int k) const {
+        SkASSERT(0 <= k && k < 4);
+        union { uint8x8_t v; uint8_t us[8]; } pun = {fVec};
+        return pun.us[k&3];
+    }
 
     // TODO as needed