[llvm-branch-commits] AArch64: Relax x16/x17 constraint on AUT in certain cases. (PR #132857)

[Peter Collingbourne via llvm-branch-commits]

https://github.com/pcc created https://github.com/llvm/llvm-project/pull/132857

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, or if traps
are requested. All other platforms use the unconstrained variant of the
instruction for selection.