insns.def: add comments to def/opt_operand.def

* insns.def (getlocal,setlocal): add comment to def/opt_operand.def

I spent some time thinking about the same optimization before
realizing it was already done.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48106 b2dd03c8-39d4-4d8f-98ff-823fe69b080e