'assoc' is not side-effect-free; constprop its pure subset

'assoc' is not side-effect-free; constprop its pure subset

Since a supplied test function can do anything, assoc is not
side-effect-free (bug#44018).  However, with only two arguments it is
pure and should be optimised accordingly.

* lisp/emacs-lisp/byte-opt.el (side-effect-free-fns): Remove 'assoc'.
(byte-optimize-assoc): Constant-propagate through 2-arg assoc calls.
* test/lisp/emacs-lisp/bytecomp-tests.el
(byte-opt-testsuite-arith-data): Add test cases.