[AArch64][GlobalISel] Fix incorrect ABI when tail call not supported (#70215)

The check for whether a tail call is supported calls
determineAssignments(), which may modify argument flags. As such, even
though the check fails and a non-tail call will be emitted, it will not
have a different (incorrect) ABI.

Fix this by operating on a separate copy of the arguments.

Fixes https://github.com/llvm/llvm-project/issues/70207.