Nullcheck elimination fix
Nullcheck elimination fix

Two problems: looking at "uses" rather than "defs" to discover new
result that is known non-null, and search needed to extend into fallthrough
basic block.

Change-Id: I11b4f5e4ce86552437ca9638c7b2728d1d7abc26

treadstoneproject/artinst,treadstoneproject/artinst,treadstoneproject/artinst,treadstoneproject/artinst,treadstoneproject/artinst