[AArch64] Add test case for preferred function alignment (NFC).

Reviewers: evandro, joelkevinjones, mcrosier

Reviewed By: joelkevinjones, mcrosier

Subscribers: mcrosier, aemerson, llvm-commits, rengolin, evandro, javed.absar, joelkevinjones, kristof.beyls

Differential Revision: https://reviews.llvm.org/D34951

llvm-svn: 307369