[AArch64] Remove unused IsDarwin & IsNotDarwin predicates (NFCI).

Reviewers: t.p.northover, rengolin

Reviewed By: t.p.northover

Subscribers: aemerson, javed.absar, llvm-commits, kristof.beyls

Differential Revision: https://reviews.llvm.org/D35266

llvm-svn: 307706