gl_shader_decompiler: Improve handling of unknown input/output attributes.

src/video_core/engines/shader_bytecode.h

@@ -74,12 +74,11 @@ union Attribute {
     enum class Index : u64 {
         Position = 7,
         Attribute_0 = 8,
+        Attribute_31 = 39,
         // This attribute contains a tuple of (~, ~, InstanceId, VertexId) when inside a vertex
         // shader, and a tuple of (TessCoord.x, TessCoord.y, TessCoord.z, ~) when inside a Tess Eval
         // shader.
         TessCoordInstanceIDVertexID = 47,
-        // TODO(bunnei): Figure out what this is used for. Super Mario Odyssey uses this.
-        Unknown_63 = 63,
     };
 
     union {