* vm.c: introduce `ruby_vm_event_enabled_flags` which represents which
  event flags are enabled before.

* vm_trace.c: do not turn off `trace_` prefix instructions because turn on
  overhead is a matter if a program repeats turn on and turn off frequently.

* iseq.c (finish_iseq_build): respect `ruby_vm_event_enabled_flags`.

* vm_insnhelper.c (vm_trace): check `ruby_vm_event_flags` and disable
  lazy trace-off technique (do not disable traces).


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@61122 b2dd03c8-39d4-4d8f-98ff-823fe69b080e