simplify one kind of degenerate prompt

simplify one kind of degenerate prompt

* module/language/tree-il/peval.scm (<operand>): Instead of having a
  `residualize?' field, have it be a use count.
  (peval): Adapt to <operand> change.  Add function to kill uses of an
  operand.  Use it in the <prompt> inliner.  Add another kind of
  degenerate prompt to elide.  We should really switch to CPS though, as
  that will allow us to contify more aggressively.

* test-suite/tests/peval.test ("partial evaluation"): Adapt (while #t
  #t) test, which was sensitive to how far the recursive inlining got.
  Add a test for the degenerate prompt elision.