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

This reverts commit a37e46323c.

It broke the game Overlord such that it hung a GCN GNU. While I don't know
how the hang happened because of its randomness and gfx corruption precedes
it, many of the shaders contain this:

out vec4 FragData[gl_MaxDrawBuffers];

src/compiler/glsl/opt_dead_builtin_varyings.cpp

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