compiler: add SYSTEM_VALUE_VARYING_COORD

Used internally in freedreno/ir3 for the vec2 value that hw passes to
shader to use as coordinate for bary.f (varying fetch) instruction.
This is not the same as SYSTEM_VALUE_FRAG_COORD.

Signed-off-by: Rob Clark <robdclark@gmail.com>

src/compiler/shader_enums.h

@@ -601,6 +601,12 @@ typedef enum
     */
    SYSTEM_VALUE_VERTEX_CNT,
 
+   /**
+    * Driver internal varying-coord, used for varying-fetch instructions.
+    * Not externally visible.
+    */
+   SYSTEM_VALUE_VARYING_COORD,
+
    SYSTEM_VALUE_MAX             /**< Number of values */
 } gl_system_value;