add SkNx::abs(), for now only implemented for Sk4f

src/core/SkNx.h

Index: src/core/SkNx.h
diff --git a/src/core/SkNx.h b/src/core/SkNx.h
index 684a31dcef1a80deb10bd5ec6f5910a1c9dd5dee..2bbd495ce50bca907407b87cc6bd315876e03a96 100644
--- a/src/core/SkNx.h
+++ b/src/core/SkNx.h
@@ -74,6 +74,8 @@ public:
         return SkNx(SkNx<N/2, T>::Max(a.fLo, b.fLo), SkNx<N/2, T>::Max(a.fHi, b.fHi));
     }
 
+    SkNx abs() const { return SkNx(fLo.abs(), fHi.abs()); }
+
     SkNx sqrt() const { return SkNx(fLo.sqrt(), fHi.sqrt()); }
     // Generally, increasing precision, increasing cost.
     SkNx rsqrt0() const { return SkNx(fLo.rsqrt0(), fHi.rsqrt0()); }
@@ -134,6 +136,8 @@ public:
     static SkNx Min(const SkNx& a, const SkNx& b) { return SkNx(SkTMin(a.fVal, b.fVal)); }
     static SkNx Max(const SkNx& a, const SkNx& b) { return SkNx(SkTMax(a.fVal, b.fVal)); }
 
+    SkNx abs() const { return SkTAbs(fVal); }
+
     SkNx  sqrt () const { return SkNx(Sqrt(fVal)); }
     SkNx rsqrt0() const { return this->sqrt().invert();  }
     SkNx rsqrt1() const { return this->rsqrt0(); }