ART: Naive NullCheck elimination in InstructionBuilder

Save a little bit of memory by not generating trivially redundant
HNullCheck instructions. This patch builds on the fact that the
InstructionBuilder now directly generates SSA form and looks at the
input of the NullCheck. For obvious cases, such as NewInstance/Array
or `this`, the NullCheck generation is completely avoided.

Bug: 28173563
Change-Id: I1fdf3b096c7a939f7b8586c02a2a6b44dfa43443