add SkNx::abs(), for now only implemented for 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 b3f509140626816367b4f34821299e44aeeb1eea..c6163b6a8034d5035d910f66fda05094a17973a2 100644
--- a/src/opts/SkNx_sse.h
+++ b/src/opts/SkNx_sse.h
@@ -176,6 +176,8 @@ public:
     static SkNx Min(const SkNx& l, const SkNx& r) { return _mm_min_ps(l.fVec, r.fVec); }
     static SkNx Max(const SkNx& l, const SkNx& r) { return _mm_max_ps(l.fVec, r.fVec); }
 
+    SkNx abs() const { return _mm_andnot_ps(_mm_set1_ps(-0.0f), fVec); }
+
     SkNx  sqrt() const { return _mm_sqrt_ps (fVec);  }
     SkNx rsqrt0() const { return _mm_rsqrt_ps(fVec); }
     SkNx rsqrt1() const { return this->rsqrt0(); }