[IR] Optimize tmp-var retrieval in interp loop.

* Buys us back most of the lost performance on default path from
  recent unboxing/tmp-var changes.
Bulk of rescue logic working in IR.

A few IR changes were needed:

* prepareForCompilation now calls prepareForInterpretation to set
  up the rescue table.
* Chained rescues only recv_exception once, loading from variable
  from then on.
Merge remote-tracking branch 'github/master'