Tidy up ir_opt.c aspects relating to the 'grail' work. In particular:

Tidy up ir_opt.c aspects relating to the 'grail' work.  In particular:

* Rewrite do_minimal_initial_iropt_BB so it doesn't do full constant folding;
  that is unnecessary expense at this point, and later passes will do it
  anyway

* do_iropt_BB: don't flatten the incoming block, because
  do_minimal_initial_iropt_BB will have run earlier and done so.  But at least
  for the moment, assert that it really is flat.

* VEX/priv/guest_generic_bb_to_IR.c create_self_checks_as_needed: generate
  flat IR so as not to fail the abovementioned assertion.

I believe this completes the target-independent aspects of this work, and also
the x86_64 specifics (of which there are very few).