LAP peephole optimisation improvements

LAP peephole optimisation improvements

- Since discardN-preserve-tos(1) and stack-set(1) have the same
  effect, treat them as equivalent in all transformations.

- Move the rule

        discardN-preserve-tos(X) discardN-preserve-tos(Y)
    --> discardN-preserve-tos(X+Y)

  from the final pass to the main iteration since it may enable
  further optimisations.

- Don't apply the rule

    goto(X) ... X: DISCARD --> DISCARD goto(Y) ... X: DISCARD Y:

  when DISCARD could be merged or deleted instead, which is even better.

- Add the rule

    OP const return -> <deleted> const return

  where OP is effect-free.

- Generalise the push-pop annihilation rule to

    PUSH(K) discard(N) -> discard(N-K), N>K
    PUSH(K) discard(N) -> <deleted>,    N=K

  to any N, not just N=1.

- Add the rule

    OP goto(X) Y: OP X: -> <deleted> Y: OP X:

  for any operation OP.

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