Manually optimize exp()

ScriptableRenderPipeline/HDRenderPipeline/Material/Lit/Resources/SubsurfaceScattering.compute

 // We can drop the constant (s / (8 * Pi)) due to the subsequent weight renormalization.
 float3 DisneyProfilePolar(float r, float3 S)
 {
+#if 0
+#else
+    #define LOG2_E 1.44269504088896340736
+    float  k = (-1.0 / 3.0) * LOG2_E;
+    float3 p = (k * r) * S;
+    float3 expOneThird = exp2(p);
+#endif
     return expOneThird + expOneThird * expOneThird * expOneThird;
 }