Ensure the graph is correctly typed.

We used to be forgiving because of HIntConstant(0) also being
used for null. We now create a special HNullConstant for such uses.

Also, we need to run the dead phi elimination twice during ssa
building to ensure the correctness.

Change-Id: If479efa3680d3358800aebb1cca692fa2d94f6e5
16 files changed