tree-optimization/104880 - update-address-taken and cmpxchg

tree-optimization/104880 - update-address-taken and cmpxchg

The following addresses optimistic non-addressable marking of
an argument of __atomic_compare_exchange_n which broke when
I added DECL_NOT_GIMPLE_REG_P since we cannot guarantee we can
rewrite it when TREE_ADDRESSABLE is unset.  Instead we have to
restore TREE_ADDRESSABLE in that case.

2022-03-11  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/104880
	* tree-ssa.cc (execute_update_address_taken): Remember if we
	optimistically made something not addressable and
	prepare to undo it.

	* g++.dg/opt/pr104880.cc: New testcase.