compile.c: add opt_aset instruction for faster Hash#[]= and Array#[]=

* compile.c (iseq_specialized_instruction): emit opt_aset instruction
  to optimize Hash#[]= and Array#[]= when called with Fixnum argument.
  [Bug #9227] [ruby-core:58956]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44136 b2dd03c8-39d4-4d8f-98ff-823fe69b080e