Fix variable aliasing bytecode miscompilation (bug#67116)

Fix variable aliasing bytecode miscompilation (bug#67116)

The compiler didn't cancel aliasing if the aliased variable was
modified in a variable binding in the same `let` that created
the alias.  For example,

 (let ((x A))
   (let ((y x)
         (z (setq x B)))
     y))

would incorrectly substitute y->x in the body form despite x being
already modified at that point, which normally should have cancelled
the aliasing.

Bug reported by Alan Mackenzie.

* lisp/emacs-lisp/byte-opt.el (byte-optimize--aliased-vars):
Now an alist that also contains the aliases; update the doc string.
* lisp/emacs-lisp/byte-opt.el (byte-optimize-form-code-walker):
* lisp/emacs-lisp/byte-opt.el (byte-optimize-let-form):
Detect aliasing early for `let`-bound variables as well.
* test/lisp/emacs-lisp/bytecomp-tests.el (bytecomp-tests--test-cases):
Add test cases.