Help the compiler save VGPR

SubsurfaceScattering.compute:

Before:
; -------- Statistics ---------------------
; SGPRs: 62 out of 104 used
; VGPRs: 42 out of 256 used
; LDS: 6416 out of 32768 bytes used
; 0 bytes scratch space used
; Instructions: 1280 ALU, 264 Control Flow, 75 TFETCH

After:
; -------- Statistics ---------------------
; SGPRs: 62 out of 104 used
; VGPRs: 41 out of 256 used
; LDS: 6416 out of 32768 bytes used
; 0 bytes scratch space used
; Instructions: 1276 ALU, 264 Control Flow, 75 TFETCH

ScriptableRenderPipeline/Core/ShaderLibrary/Common.hlsl

 #define HALF_PI     1.57079632679
 #define INV_HALF_PI 0.636619772367
 #define INFINITY    asfloat(0x7F800000)
+#define LOG2_E      1.44269504089
 
 #define FLT_EPSILON 1.192092896e-07 // Smallest positive number, such that 1.0 + FLT_EPSILON != 1.0
 #define FLT_MIN     1.175494351e-38 // Minimum representable positive floating-point number

ScriptableRenderPipeline/Core/ShaderLibrary/CommonMaterial.hlsl

     // In theory, we should modify the thickness by the inverse of the radius scale of the profile.
     // thickness /= radiusScale;
 
+#if 0
+#else
+    // Help the compiler.
+    float  k = (-1.0 / 3.0) * LOG2_E;
+    float3 p = (k * thickness) * S;
+    float3 expOneThird = exp2(p);
+#endif
 
     // Premultiply & optimize: T = (1/4 * A) * (e^(-t * S) + 3 * e^(-1/3 * t * S))
     return volumeAlbedo * (expOneThird * expOneThird * expOneThird + 3 * expOneThird);

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
+    // Help the compiler.
+    float  k = (-1.0 / 3.0) * LOG2_E;
+    float3 p = (k * r) * S;
+    float3 expOneThird = exp2(p);
+#endif
     return expOneThird + expOneThird * expOneThird * expOneThird;
 }