array.c: optimized equality

* array.c (rb_ary_index, rb_ary_rindex): use optimized equality to
  improve performance.  [Feature #8820]
* vm_insnhelper.c (rb_equal_opt): optimized equality function.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42704 b2dd03c8-39d4-4d8f-98ff-823fe69b080e