[AArch64][PAC] Fix creating check instructions for BBs without an epilog

`AArch64PAuth::checkAuthenticatedRegister()` splits the basic block
containing the tail call instruction to add check instructions, assuming
at least one more instruction before the call. This assumption is
incorrect in cases where some execution paths lead to the termination
block without creating the stack frame. This patch rearranges the
creation of the checks so that the prior splitting is not required.