* bignum.c (rb_big_cmp): Specialize a comparison to zero.

* ext/bigdecimal/bigdecimal.c (is_negative): Use rb_big_cmp instead of
  RBIGNUM_NEGATIVE_P.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44910 b2dd03c8-39d4-4d8f-98ff-823fe69b080e