Re-apply "glsl: don't try to lower non-gl builtins as if they were gl_FragData"

If a shader has an output array, it will get treated as though it were
gl_FragData and rewritten into gl_out_FragData instances. We only want
this to happen on the actual gl_FragData and not everything else.

This is a small part of the problem pointed out by the below bug.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=96765
Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/compiler/glsl/opt_dead_builtin_varyings.cpp

@@ -85,7 +85,8 @@ public:
    {
       ir_variable *var = ir->variable_referenced();
 
-      if (!var || var->data.mode != this->mode || !var->type->is_array())
+      if (!var || var->data.mode != this->mode || !var->type->is_array() ||
+          !is_gl_identifier(var->name))
          return visit_continue;
 
       /* Only match gl_FragData[], not gl_SecondaryFragDataEXT[] */