Optimise let and let* whose body is constant or the last variable

Optimise let and let* whose body is constant or the last variable

Simplify  (let ((X1 E1) ... (Xn En)) Xn)
      =>  (progn E1 ... En)

and       (let* ((X1 E1) ... (Xn En)) Xn)
      =>  (let* ((X1 E1) ... (Xn-1 En-1)) En)

and similarly the case where the body is a constant, extending a
previous optimisation that only applied to the constant nil.
This reduces the number of bound variables, shortens the code, and
enables further optimisations.

* lisp/emacs-lisp/byte-opt.el (byte-optimize-letX): Rewrite using
`pcase` and add the aforementioned transformations.
* test/lisp/emacs-lisp/bytecomp-tests.el (bytecomp-tests--test-cases):
Add test cases.