I think when this code was written, basic blocks were always ended by a
control flow instruction or an end-of-thread message. That's no longer
the case, and removing this restriction actually helps things:

   instructions in affected programs: 7267 -> 7244 (-0.32%)
   helped: 4

   total cycles in shared programs: 66559580 -> 66431900 (-0.19%)
   cycles in affected programs: 28310152 -> 28182472 (-0.45%)
   helped: 9577
   HURT: 879

   GAINED: 2

The addition of the is_control_flow() checks is not a functional change,
since the add_insts_from_block() does not put them in the list of
instructions to schedule. I plan to change this in a later patch.

Reviewed-by: Francisco Jerez <currojerez@riseup.net>