middle-end/108786 - add bitmap_clear_first_set_bit

This adds bitmap_clear_first_set_bit and uses it where previously
bitmap_clear_bit followed bitmap_first_set_bit.  The advantage
is speeding up the search and avoiding to clobber ->current.

	PR middle-end/108786
	* bitmap.h (bitmap_clear_first_set_bit): New.
	* bitmap.cc (bitmap_first_set_bit_worker): Rename from
	bitmap_first_set_bit and add optional clearing of the bit.
	(bitmap_first_set_bit): Wrap bitmap_first_set_bit_worker.
	(bitmap_clear_first_set_bit): Likewise.
	* df-core.cc (df_worklist_dataflow_doublequeue): Use
	bitmap_clear_first_set_bit.
	* graphite-scop-detection.cc (scop_detection::merge_sese):
	Likewise.
	* sanopt.cc (sanitize_asan_mark_unpoison): Likewise.
	(sanitize_asan_mark_poison): Likewise.
	* tree-cfgcleanup.cc (cleanup_tree_cfg_noloop): Likewise.
	* tree-into-ssa.cc (rewrite_blocks): Likewise.
	* tree-ssa-dce.cc (simple_dce_from_worklist): Likewise.
	* tree-ssa-sccvn.cc (do_rpo_vn_1): Likewise.