tree-optimization/100398 - avoid DSE of control flow stmt

The following makes sure to preserve control altering stmts
when removing trivially dead stmts in DSE.

2021-05-04  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/100398
	* tree-ssa-dse.c (pass_dse::execute): Preserve control
	altering stmts.

	* gcc.dg/torture/pr100398.c: New testcase.