When removing a phi node, add the def statement for the (previously)
used temps to the worklist. These statements might now be eligible for
further optimizations (specifically removal when there are no uses
left).

Change-Id: I05d7c7bc0a243d328b5f9d1c2dcc53a10bd7491d
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>