[AArch64] Enable UseAA globally in the AArch64 backend

This is similar to D69796 from the ARM backend. We remove the UseAA
feature, enabling it globally in the AArch64 backend. This should in
general be an improvement allowing the backend to reorder more
instructions in scheduling and codegen, and enabling it by default helps
to improve the testing of the feature, not making it cpu-specific. A
debugging option is added instead for testing.

Differential Revision: https://reviews.llvm.org/D98781