[AArch64] Add custom lowering for ISD::ABS

Instead of trying to pattern match the code produced by ISD::ABS expansion, just custom legalize ISD::ABS to the desired sequence.

The one test change is because a DAG combine for (neg (abs)) is no longer firing because ISD::ABS is now Custom instead of Expand.

Differential Revision: https://reviews.llvm.org/D92154