vm_insnhelper.c: optimize for loop

* vm_insnhelper.c (vm_call_iseq_setup_normal): simple for loop
  condition optimization. this area shows up as a hotspot in VM
  profiles.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44285 b2dd03c8-39d4-4d8f-98ff-823fe69b080e