Add new/delete to struct occurence

This adds an example how to use new/delete operators to pool
allocated objects.

2020-06-04  Jonathan Wakely  <jwakely@redhat.com>

	* alloc-pool.h (object_allocator::remove_raw): New.
	* tree-ssa-math-opts.c (struct occurrence): Use NSMDI.
	(occurrence::occurrence): Add.
	(occurrence::~occurrence): Likewise.
	(occurrence::new): Likewise.
	(occurrence::delete): Likewise.
	(occ_new): Remove.
	(insert_bb): Use new occurence (...) instead of occ_new.
	(register_division_in): Likewise.
	(free_bb): Use delete occ instead of manually removing
	from the pool.