glsl: Don't attempt to do dead varying elimination on gl_LastFragData arrays.

Apparently this pass can only handle elimination of a single built-in
fragment output array, so the presence of gl_LastFragData (which it
wouldn't split correctly anyway) could prevent it from splitting the
actual gl_FragData array.  Just match gl_FragData by name since it's
the only built-in it can handle.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/compiler/glsl/opt_dead_builtin_varyings.cpp

@@ -89,9 +89,10 @@ public:
           !is_gl_identifier(var->name))
          return visit_continue;
 
-      /* Only match gl_FragData[], not gl_SecondaryFragDataEXT[] */
+      /* Only match gl_FragData[], not gl_SecondaryFragDataEXT[] or
-      if (this->find_frag_outputs && var->data.location == FRAG_RESULT_DATA0 &&
+       * gl_LastFragData[].
-          var->data.index == 0) {
+       */
+      if (this->find_frag_outputs && strcmp(var->name, "gl_FragData") == 0) {
          this->fragdata_array = var;
 
          ir_constant *index = ir->array_index->as_constant();