tree-optimization/99912 - delete trivially dead stmts during DSE

tree-optimization/99912 - delete trivially dead stmts during DSE

DSE performs a backwards walk over stmts removing stores but it
leaves removing resulting dead SSA defs to later passes.  This
eats into its own alias walking budget if the removed stores kept
loads live.  The following patch adds removal of trivially dead
SSA defs which helps in this situation and reduces the amount of
garbage followup passes need to deal with.

2021-04-28  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/99912
	* tree-ssa-dse.c (dse_dom_walker::m_need_cfg_cleanup): New.
	(dse_dom_walker::todo): Likewise.
	(dse_dom_walker::dse_optimize_stmt): Move VDEF check to the
	caller.
	(dse_dom_walker::before_dom_children): Remove trivially
	dead SSA defs and schedule CFG cleanup if we removed all
	PHIs in a block.
	(pass_dse::execute): Get TODO as computed by the DOM walker
	and return it.  Wipe dominator info earlier.

	* gcc.dg/pr95580.c: Disable DSE.
	* gcc.dg/Wrestrict-8.c: Place a use after each memcpy.
	* c-c++-common/ubsan/overflow-negate-3.c: Make asms volatile
	to prevent them from being removed.
	* c-c++-common/ubsan/overflow-sub-4.c: Likewise.