tree-optimization/100434 - DSE aggregate call LHS

tree-optimization/100434 - DSE aggregate call LHS

This makes DSE consider aggregate LHS of calls as dead, for pure
or const calls the whole stmt and for others by removing the LHS.

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

	PR tree-optimization/100434
	* tree-ssa-dse.c (initialize_ao_ref_for_dse): Handle
	call LHS.
	(dse_optimize_stmt): Handle call LHS by dropping the
	LHS or the whole call if it doesn't have other
	side-effects.
	(pass_dse::execute): Adjust.

	* gcc.dg/tree-ssa/ssa-dse-43.c: New testcase.