[AArch64] Enable Cortex-A55 schedmodel
The model was committed in 4b8ade837e36b7f0181ce86fc23f33851d0fdd35
but not yet enabled to allow for a few fix ups. This adds a few
of these fixes, and also a LLVM MCA test to check most instructions.
While I do have plans to look into some more tuning, it's time to
enable this as it better than using the A53 schedule.
Differential Revision: https://reviews.llvm.org/D88017