LAP optimiser: bind local variables instead of mutating them

LAP optimiser: bind local variables instead of mutating them

This is a refactoring step: there is no change in how the optimiser
works.

* lisp/emacs-lisp/byte-opt.el (byte-optimize-lapcode):
Instead of re-using local variables through mutation, bind them at
point of use.  This ensures that there is no value leakage by mistake
and actually reduces the static size of the bytecode of this function
somewhat.
The lousy variable names (tmp, tmp2 etc) are retained but
can at least now be changed into something more descriptive.