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.
There are already a lot of command lines added to this test, presumably to attempt to get it to work in the same way as it did in the past. -pre-RA-sched=linearize -enable-misched=false -disable-post-ra. Without those flags this performs the same with and without UseAA.
In general, UseAA can give more scheduling freedom and the compiler is perfectly able to shoot itself in the foot with that extra freedom. The opposite can be true too though, where it does help things, and it should help more than it hinders. And it's already enabled for certain CPU's. This just makes it global.