[FastISel][AArch64] Emit immediate version of icmp (subs) for null pointer check.

This is a minor change to use the immediate version when the operand is a null
value. This should get rid of an unnecessary 'mov' instruction in debug
builds and align the code more with the one generated by SelectionDAG.

This fixes rdar://problem/18785125.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220713 91177308-0d34-0410-b5e6-96231b3b80d8