AArch64: Don't use RETA[AB] when ShadowCallStack is enabled.

When returning from a function with both SCS and PAC-RET enabled, we need to
authenticate the return address from the stack and then load from the SCS,
but this was happening in the reverse order when RETA[AB] were being used.
Fix it by disabling the use of RETA[AB] when SCS is enabled.

Fixes pr58072.

Differential Revision: https://reviews.llvm.org/D134931