check trace flags at loading [Bug #14702]

	* iseq.c (iseq_init_trace): at ISeq loading time, we need to check
	  `ruby_vm_event_enabled_flags` to turn on trace instructions.
	  Seprate this checking code from `finish_iseq_build()` and make
	  new function. `iseq_ibf_load()` calls this funcation after loading.

	* test/ruby/test_iseq.rb: add a test for this fix.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_5@64997 b2dd03c8-39d4-4d8f-98ff-823fe69b080e