Fix bitmap registration of overheads.

2019-05-07  Martin Liska  <mliska@suse.cz>

	* bitmap.c (bitmap_register): Come up with
	alloc_descriptor_max_uid and assign it for
	a new bitmap.
	(register_overhead): Use get_descriptor as
	a descriptor.
	(release_overhead): New.
	(bitmap_elem_to_freelist): Call it.
	(bitmap_elt_clear_from): Likewise.
	(bitmap_obstack_free): Likewise.
	(bitmap_move): Sensitively release memory.
	* bitmap.h (struct GTY): Add alloc_descriptor and padding.
	(bitmap_initialize): Initialize alloc_descriptor to zero.
	* tree-ssa-pre.c (do_hoist_insertion): Use bitmap_move.

From-SVN: r270942