tgsi: Declare a LOOP register.

The only valid usage for LOOP/ENDLOOP instructions
is LOOP[0] as a destination register.
The only valid usage for the remaining instructions
is LOOP[0].x as an indirect register.

src/gallium/include/pipe/p_shader_tokens.h

@@ -78,6 +78,7 @@ enum tgsi_file_type {
    TGSI_FILE_SAMPLER     =5,
    TGSI_FILE_ADDRESS     =6,
    TGSI_FILE_IMMEDIATE   =7,
+   TGSI_FILE_LOOP        =8,
    TGSI_FILE_COUNT      /**< how many TGSI_FILE_ types */
 };