LAP peephole optimiser improvementsa

LAP peephole optimiser improvementsa

* lisp/emacs-lisp/byte-opt.el (byte-optimize-lapcode):
Make the improvements:

- Add the rule

    stack-ref(X) discardN-preserve-tos(Y)
    --> discard(Y) stack-ref(X-Y),                X≥Y
        discard(X) discardN-preserve-tos(Y-X-1),  X<Y

  with the usual equivalences:

    stack-set(1) = discardN-preserve-tos(1)
    stack-ref(0) = dup
    discard(0) = discardN-preserve-tos(0) = no-op

  This rule hoists stack reduction to where it is more likely to be
  exploited further, may reduce the op size through smaller
  immediates, and sometimes removes either or both operations
  outright.

  The rule is inhibited by an immediately following `return` op
  because other rules will produce better code in that case.

- Add the rule

    (discardN-preserve-tos|dup) OP return --> OP return

  where OP is a unary operation such as `not` or `car`.

- Generalise a previous rule to

    NOEFFECT PRODUCER return  -->  PRODUCER return

  where PRODUCER is now any op that pushes a value without looking at
  the stack: const, varref, point etc.