Propagate aliased lexical variables in byte compiler

Replace uses of a variable aliasing another variable with that aliased
variable, to allow for variable removal when possible.  This also
enables opportunities for other optimisations.  Example:

 (let ((y x)) (f y)) => (f x)

The optimisation is only performed if both aliased and aliasing
variables are lexically bound.  Shadowing bindings are α-renamed when
necessary for correctness.  Example:

   (let* ((b a) (a EXPR)) (f a b))
=> (let* ((a{new} EXPR)) (f a{new} a))

* lisp/emacs-lisp/byte-opt.el (byte-optimize--aliased-vars): New.
(byte-optimize-form-code-walker): Cancel aliasing upon mutation.
(byte-optimize--rename-var-body, byte-optimize--rename-var): New.
(byte-optimize-let-form): Add the optimisation.
* test/lisp/emacs-lisp/bytecomp-tests.el (bytecomp-tests--test-cases):
Add relevant test cases.

test/lisp/emacs-lisp/bytecomp-tests.el

@@ -551,6 +551,50 @@
     (let ((n 0))
       (list (mapcar (lambda (x) (assoc (setq n (1+ n)) nil)) '(a "nil"))
             n))
+
+    ;; Exercise variable-aliasing optimisations.
+    (let ((a (list 1)))
+      (let ((b a))
+        (let ((a (list 2)))
+          (list a b))))
+
+    (let ((a (list 1)))
+      (let ((a (list 2))
+            (b a))
+        (list a b)))
+
+    (let* ((a (list 1))
+           (b a)
+           (a (list 2)))
+      (condition-case a
+          (list a b)
+        (error (list 'error a b))))
+
+    (let* ((a (list 1))
+           (b a)
+           (a (list 2)))
+      (condition-case a
+          (/ 0)
+        (error (list 'error a b))))
+
+    (let* ((a (list 1))
+           (b a)
+           (a (list 2))
+           (f (list (lambda (x) (list x a)))))
+      (funcall (car f) 3))
+
+    (let* ((a (list 1))
+           (b a)
+           (f (list (lambda (x) (setq a x)))))
+      (funcall (car f) 3)
+      (list a b))
+
+    (let* ((a (list 1))
+           (b a)
+           (a (list 2))
+           (f (list (lambda (x) (setq a x)))))
+      (funcall (car f) 3)
+      (list a b))
     )
   "List of expressions for cross-testing interpreted and compiled code.")