Change coerce_to to rb_to_int to get expected error

Author: rwstauner

spec/tags/core/matchdata/byteoffset_tags.txt

@@ -155,7 +155,7 @@ def backref_from_arg(index)
       end
       names_to_backref[index.to_sym].last
     else
-      Truffle::Type.coerce_to(index, Integer, :to_int)
+      Primitive.rb_to_int(index)
     end
   end
 end