object-size: Use simple_dce_from_worklist in object-size pass

While trying to see if there was a way to improve object-size pass
to use the ranger (for pointer plus), I noticed that it leaves around
the statement containing __builtin_object_size if it was reduced to a constant.
This fixes that by using simple_dce_from_worklist.

Bootstrapped and tested on x86_64-linux-gnu.

gcc/ChangeLog:

	* tree-object-size.cc (object_sizes_execute): Mark lhs for maybe dceing
	if doing a propagate. Call simple_dce_from_worklist.

Signed-off-by: Andrew Pinski <quic_apinski@quicinc.com>