Revert "* encoding.c (enc_find): simplified with RB_TYPE_P."

This reverts commit r32817.
It has less code on the surface, but it needs more condition.
Moreover it doesn't need !.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@32822 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

encoding.c

@@ -1054,7 +1054,7 @@ static VALUE
 enc_find(VALUE klass, VALUE enc)
 {
     int idx;
-    if (!RB_TYPE_P(enc, T_DATA) && is_data_encoding(enc))
+    if (!SPECIAL_CONST_P(enc) && BUILTIN_TYPE(enc) == T_DATA && is_data_encoding(enc))
 	return enc;
     idx = str_to_encindex(enc);
     if (idx == UNSPECIFIED_ENCODING) return Qnil;