Perform reverse program order walk for GIMPLE DSE

Perform reverse program order walk for GIMPLE DSE

The following changes the post-dominator domwalk done by GIMPLE DSE
to a reverse program order walk.  This enables 2% more stmts do be
DSEd during bootstrap and in particular for testcases like the one
added where it is important to visit post dominators in a particular
order.

2021-05-03  Richard Biener  <rguenther@suse.de>

	* tree-ssa-dse.c: Do not include domwalk.h but cfganal.h.
	(dse_dom_walker): Remove.
	(dse_dom_walker::dse_optimize_stmt): Rename...
	(dse_optimize_stmt): ... to this, pass in live_bytes sbitmap.
	(dse_dom_walker::before_dom_children): Inline ...
	(pass_dse::execute): ... here.  Perform a reverse program
	order walk.

	* gcc.dg/tree-ssa/ssa-dse-41.c: New testcase.