[middle-end, obvious] Remove dead error_mark_node check

[Paolo Carlini]

Hi,

today noticed this nit too: when ret_expr == error_mark_node 
gimplify_return_expr immediatey returns GS_ERROR.

Thanks, Paolo.

//////////////////


2017-12-20  Paolo Carlini  <paolo.carl...@oracle.com>

        * gimplify.c (gimplify_return_expr): Remove dead error_mark_node check.
Index: gimplify.c
===================================================================
--- gimplify.c  (revision 255855)
+++ gimplify.c  (working copy)
@@ -1499,10 +1499,9 @@ gimplify_return_expr (tree stmt, gimple_
   if (ret_expr == error_mark_node)
     return GS_ERROR;
 
   if (!ret_expr
-      || TREE_CODE (ret_expr) == RESULT_DECL
-      || ret_expr == error_mark_node)
+      || TREE_CODE (ret_expr) == RESULT_DECL)
     {
       maybe_add_early_return_predict_stmt (pre_p);
       greturn *ret = gimple_build_return (ret_expr);
       gimple_set_no_warning (ret, TREE_NO_WARNING (stmt));