This takes advanatage of the CBZ and CBNZ instruction to further optimize the
common null check pattern into a single instruction.
This is related to rdar://problem/18358882.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217972 91177308-0d34-0410-b5e6-96231b3b80d8