AArch64: Relax x16/x17 constraint on AUT in certain cases.
On most operating systems, the x16 and x17 registers are not special,
so there is no benefit, and only a code size cost, to constraining AUT to
only using them. Therefore, adjust the backend to only use the AUT pseudo
(renamed AUTx16x17 for clarity) on Darwin platforms. All other platforms
use an unconstrained variant of the pseudo, AUTxMxN, for selection.
Reviewers: ahmedbougacha, kovdan01, atrosinenko
Reviewed By: atrosinenko
Pull Request: https://github.com/llvm/llvm-project/pull/132857
