Correctly eliminate duplicate cases in switch compilation

Correctly eliminate duplicate cases in switch compilation

Fix code mistakes that prevented the correct elimination of duplicated
cases when compiling a `cond' form to a switch bytecode, as in

  (cond ((eq x 'a) 1)
        ((eq x 'b) 2)
        ((eq x 'a) 3)   ; should be elided
        ((eq x 'c) 4))

Sometimes, this caused the bytecode to use the wrong branch (bug#35770).

* lisp/emacs-lisp/bytecomp.el (byte-compile-cond-vars): Return obj2 eval'ed.
(byte-compile-cond-jump-table-info):
Discard redundant condition.  Use `obj2' as evaluated.
Discard duplicated cases instead of failing the table generation.
* test/lisp/emacs-lisp/bytecomp-tests.el (toplevel): Require subr-x.
(byte-opt-testsuite-arith-data, bytecomp-test--switch-duplicates): Test.