Fix some issues with folded AArch64 features
In #91608 the `fp` feature was removed for AArch64 and folded into the `neon` feature, however disabling the `neon` feature doesn't actually disable the `fp` feature. If my understanding on that thread is correct it should do.
While doing this, I also noticed that disabling some features would disable features that it shouldn't. For instance enabling `sve` will enable `neon`, however, when disabling `sve` it would then also disable `neon`, I wouldn't expect disabling `sve` to also disable `neon`.
cc `@workingjubilee`
One extension worth noting is the use of revisions as custom prefixes for
FileCheck. If your codegen test has different behavior based on the chosen
target or different compiler flags that you want to exercise, you can use a
revisions annotation, like so:
After specifying those variations, you can write different expected, or
explicitly unexpected output by using <prefix>-SAME: and <prefix>-NOT:,
like so: