RTL slot allocator uses more precise, correct liveness information

RTL slot allocator uses more precise, correct liveness information

* module/language/cps/dfg.scm (control-point?): New interface, replaces
  branch?.
  (dead-after-def?, dead-after-use?, dead-after-branch?): Remove these.
  The first one was fine; dead-after-use? was conservative but OK; but
  dead-after-branch? was totally bogus.  Instead we use precise liveness
  information in the allocator.

* module/language/cps/slot-allocation.scm ($allocation): Remove "def"
  and "dead" slots.  We'll communicate liveness information in some
  other way to the compiler.
  (allocate-slots): Rework to use precise liveness information.