ShaderGen: Register id 255 is special and is hardcoded to return 0 (SR_ZERO).

src/video_core/renderer_opengl/gl_shader_decompiler.cpp

@@ -220,6 +220,8 @@ private:
 
     /// Generates code representing a temporary (GPR) register.
     std::string GetRegister(const Register& reg, unsigned elem = 0) {
+        if (reg == Register::ZeroIndex)
+            return "0";
         if (stage == Maxwell3D::Regs::ShaderStage::Fragment && reg < 4) {
             // GPRs 0-3 are output color for the fragment shader
             return std::string{"color."} + "rgba"[(reg + elem) & 3];