ART: Do not eagerly type LoopPhi [null, ...]

ReferenceTypePropagation would eagerly set the type of each loop phi
to the type of the first input prior to beginning the fix-point
iteration. While this does make the algorithm converge faster, it
should not be applied when the first input is a NullConstant becuase
that sets the type of the phi and all dependent instructions to Object.

Bug: 25899441
Change-Id: Iff1ed26a63fe4332eaf88d9ca171e287f10ba1a6
3 files changed