LunarGOO: Correctly handle phis for empty loop exit block

[greg-lunarg]

If empty loop exit block precedes loop merge block for simple conditional
or induction loop, create phi copies before loop begin and right before
loop end. Constant phi copies can be avoided at loop end.

[johnkslang]

See #2. This merge request is an expansion of the ad hoc method of handling phi. Given this code is already written, I can review it and merge it in. However, if another special case shows up, I would encourage doing the experiment with the alternative full solution before deciding to further incrementally pursue the ad hoc approach.