LAP optimiser: more stack reduction hoisting

LAP optimiser: more stack reduction hoisting

Hoisting stack reduction ops allows them to coalesce and/or cancel out
pushing ops, and for useful operations to sink and combine, such as
not + goto-if-[not-]nil.

* lisp/emacs-lisp/byte-opt.el (byte-optimize-lapcode):
Add the rule

  UNARY discardN-preserve-tos --> discardN-preserve-tos UNARY

where UNARY pops and pushes one value.

Generalise the rule

  const discardN-preserve-tos --> discardN const

to any 0-ary op, not just const: varref, point, etc.