Move float<->byte conversions into Sk4f.

src/opts/SkNx_sse.h

Index: src/opts/SkNx_sse.h
diff --git a/src/opts/SkNx_sse.h b/src/opts/SkNx_sse.h
index e165f587370c2af26caeb75b364a478c6abd3141..093cd4c14cfcf58bbb35c1b726082db8d461fd40 100644
--- a/src/opts/SkNx_sse.h
+++ b/src/opts/SkNx_sse.h
@@ -155,9 +155,30 @@ public:
     SkNf() {}
     explicit SkNf(float val)           : fVec( _mm_set1_ps(val) ) {}
     static SkNf Load(const float vals[4]) { return _mm_loadu_ps(vals); }
+
+    static SkNf FromBytes(const uint8_t bytes[4]) {
+        __m128i fix8 = _mm_cvtsi32_si128(*(const int*)bytes);
+    #if SK_CPU_SSE_LEVEL >= SK_CPU_SSE_LEVEL_SSSE3
+        const char _ = ~0;  // Zero these bytes.
+        __m128i fix8_32 = _mm_shuffle_epi8(fix8, _mm_setr_epi8(0,_,_,_, 1,_,_,_, 2,_,_,_, 3,_,_,_));
+    #else
+        __m128i fix8_16 = _mm_unpacklo_epi8 (fix8,    _mm_setzero_si128()),
+                fix8_32 = _mm_unpacklo_epi16(fix8_16, _mm_setzero_si128());
+    #endif
+        return SkNf(_mm_cvtepi32_ps(fix8_32));
+        // TODO: use _mm_cvtepu8_epi32 w/SSE4.1?
+    }
+
     SkNf(float a, float b, float c, float d) : fVec(_mm_setr_ps(a,b,c,d)) {}
 
     void store(float vals[4]) const { _mm_storeu_ps(vals, fVec); }
+    void toBytes(uint8_t bytes[4]) const {
+        __m128i fix8_32 = _mm_cvttps_epi32(fVec),
+                fix8_16 = _mm_packus_epi16(fix8_32, fix8_32),
+                fix8    = _mm_packus_epi16(fix8_16, fix8_16);
+        *(int*)bytes = _mm_cvtsi128_si32(fix8);
+        // TODO: use _mm_shuffle_epi8 w/SSSE3?
+    }
 
     SkNi<4, int> castTrunc() const { return _mm_cvttps_epi32(fVec); }