radeonsi: implement gl_SampleMaskIn

Reviewed-by: Glenn Kennard <glenn.kennard@gmail.com>

docs/GL3.txt

@@ -106,7 +106,7 @@ GL 4.0, GLSL 4.00:
   - Enhanced textureGather                             DONE (r600, radeonsi)
   - Geometry shader instancing                         DONE (r600)
   - Geometry shader multiple streams                   DONE ()
-  - Enhanced per-sample shading                        DONE (r600)
+  - Enhanced per-sample shading                        DONE (r600, radeonsi)
   - Interpolation functions                            DONE (r600)
   - New overload resolution rules                      DONE
   GL_ARB_gpu_shader_fp64                               DONE (nvc0, softpipe)

src/gallium/drivers/radeonsi/si_shader.c

@@ -643,6 +643,10 @@ static void declare_system_value(
 		break;
 	}
 
+	case TGSI_SEMANTIC_SAMPLEMASK:
+		value = LLVMGetParam(radeon_bld->main_fn, SI_PARAM_SAMPLE_COVERAGE);
+		break;
+
 	default:
 		assert(!"unknown system value");
 		return;