[IR] Clean up ToAryInstr

* Got rid of the useless dontToAryArrays flag in ToAryInstr
  I think I added it to support inlining of closures, when
  YieldInstr is converted to ToAryInstr. However, that code needs
  some fixing anyway, and it is better to fix it rather than
  hack up ToAryInstr.

* No change in rubyspec results in IR mode.