Remove overly-promiscuous SkNx syntax sugar.

I haven't figured out a pithy way to have these apply to only classes
originating from SkNx, so let's just remove them.  There aren't too
many use cases, and it's not really any less readable without them.

Semantically, this is a no-op.

BUG=skia:

Review URL: https://codereview.chromium.org/1167153002

bench/PMFloatBench.cpp

@@ -121,10 +121,10 @@ struct PMFloatGradientBench : public Benchmark {
                 fDevice[i+1] = SkPMFloat(b).trunc();
                 fDevice[i+2] = SkPMFloat(c).trunc();
                 fDevice[i+3] = SkPMFloat(d).trunc();
-                a += dcdx4;
-                b += dcdx4;
-                c += dcdx4;
-                d += dcdx4;
+                a = a + dcdx4;
+                b = b + dcdx4;
+                c = c + dcdx4;
+                d = d + dcdx4;
             }
         }
     }

src/core/SkGeometry.cpp

@@ -1284,7 +1284,7 @@ SkPoint SkConic::evalAt(SkScalar t) const {
     Sk2s numer = quad_poly_eval(A, B, C, tt);
 
     B = times_2(ww - one);
-    A = -B;
+    A = Sk2s(0)-B;
     Sk2s denom = quad_poly_eval(A, B, one, tt);
 
     return to_point(numer / denom);

src/core/SkNx.h

@@ -260,17 +260,6 @@ protected:
 
 }  // namespace
 
-// Generic syntax sugar that should work equally well for all implementations.
-template <typename T> T operator - (const T& l) { return T(0) - l; }
-
-template <typename L, typename R> L& operator += (L& l, const R& r) { return (l = l + r); }
-template <typename L, typename R> L& operator -= (L& l, const R& r) { return (l = l - r); }
-template <typename L, typename R> L& operator *= (L& l, const R& r) { return (l = l * r); }
-template <typename L, typename R> L& operator /= (L& l, const R& r) { return (l = l / r); }
-
-template <typename L> L& operator <<= (L& l, int bits) { return (l = l << bits); }
-template <typename L> L& operator >>= (L& l, int bits) { return (l = l >> bits); }
-
 // Include platform specific specializations if available.
 #ifndef SKNX_NO_SIMD
     #if SK_CPU_SSE_LEVEL >= SK_CPU_SSE_LEVEL_SSE2

src/core/SkRect.cpp

@@ -72,12 +72,12 @@ bool SkRect::setBoundsCheck(const SkPoint pts[], int count) {
             count -= 2;
         }
         accum = max = min;
-        accum *= Sk4s(0);
+        accum = accum * Sk4s(0);
 
         count >>= 1;
         for (int i = 0; i < count; ++i) {
             Sk4s xy = Sk4s::Load(&pts->fX);
-            accum *= xy;
+            accum = accum * xy;
             min = Sk4s::Min(min, xy);
             max = Sk4s::Max(max, xy);
             pts += 2;

src/core/SkScan_Hairline.cpp

@@ -233,7 +233,7 @@ static void hairquad(const SkPoint pts[3], const SkRegion* clip,
     Sk2s B = Sk2s::Load(&coeff[1].fX);
     Sk2s C = Sk2s::Load(&coeff[2].fX);
     for (int i = 1; i < lines; ++i) {
-        t += dt;
+        t = t + dt;
         ((A * t + B) * t + C).store(&tmp[i].fX);
     }
     tmp[lines] = pts[2];
@@ -241,7 +241,7 @@ static void hairquad(const SkPoint pts[3], const SkRegion* clip,
 }
 
 static inline Sk2s abs(const Sk2s& value) {
-    return Sk2s::Max(value, -value);
+    return Sk2s::Max(value, Sk2s(0)-value);
 }
 
 static inline SkScalar max_component(const Sk2s& value) {
@@ -298,7 +298,7 @@ static void hair_cubic(const SkPoint pts[4], const SkRegion* clip, SkBlitter* bl
 
     SkPoint coeff[4];
     SkCubicToCoeff(pts, coeff);
-    
+
     const Sk2s dt(SK_Scalar1 / lines);
     Sk2s t(0);
 
@@ -311,7 +311,7 @@ static void hair_cubic(const SkPoint pts[4], const SkRegion* clip, SkBlitter* bl
     Sk2s C = Sk2s::Load(&coeff[2].fX);
     Sk2s D = Sk2s::Load(&coeff[3].fX);
     for (int i = 1; i < lines; ++i) {
-        t += dt;
+        t = t + dt;
         (((A * t + B) * t + C) * t + D).store(&tmp[i].fX);
     }
     tmp[lines] = pts[3];

src/effects/gradients/SkRadialGradient.cpp

@@ -320,8 +320,8 @@ void shadeSpan_radial_clamp2(SkScalar sfx, SkScalar sdx, SkScalar sfy, SkScalar
 
         for (int i = 0; i < (count >> 2); ++i) {
             Sk4f dist = Sk4f::Min(fast_sqrt(R), max);
-            R += dR;
-            dR += ddR;
+            R = R + dR;
+            dR = dR + ddR;
 
             int fi[4];
             dist.castTrunc().store(fi);

tests/SkNxTest.cpp

@@ -46,7 +46,7 @@ static void test_Nf(skiatest::Reporter* r) {
     assert_eq(a*b-b, 6, 12, 20, 30);
     assert_eq((a*b).sqrt(), 3, 4, 5, 6);
     assert_eq(a/b, 1, 1, 1, 1);
-    assert_eq(-a, -3, -4, -5, -6);
+    assert_eq(SkNf<N,T>(0)-a, -3, -4, -5, -6);
 
     SkNf<N,T> fours(4);