Avoid unused sbitmap in update_ssa

Avoid unused sbitmap in update_ssa

The following avoids copying and using blocks_to_update to
the interesting_blocks sbitmap when doing update_ssa as it is
unused besides the redundant query in the domwalk.

	* tree-into-ssa.cc (rewrite_update_dom_walker::before_dom_children):
	Do not look at interesting_blocks which is a copy of
	blocks_to_update.
	(update_ssa): Do not initialize it.
	(pass_build_ssa::execute): Set interesting_blocks to NULL
	after releasing it.