Use const for some function arguments.

gcc/ChangeLog:

2020-02-04  Martin Liska  <mliska@suse.cz>

	PR c/92472
	* alloc-pool.h: Use const for some arguments.
	* bitmap.h: Likewise.
	* mem-stats.h: Likewise.
	* sese.h (get_entry_bb): Likewise.
	(get_exit_bb): Likewise.