[AArch64] Add AArch64Subtarget::isFusion function.

Summary:
isFusion returns true if the subtarget supports any kind of instruction
fusion, similar to ARMSubtarget::isFusion. This was suggested in D34142.

This changes the current behavior slightly, because the macro fusion mutation 
is now added to the PostRA MachineScheduler in case the subtarget supports
any kind of fusion. I think that makes sense because if the PostRA 
MachineScheduler is run, there is potential that instructions scheduled back to 
back are re-scheduled.

Reviewers: evandro, t.p.northover, joelkevinjones, joel_k_jones, steleman

Reviewed By: joelkevinjones

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

Differential Revision: https://reviews.llvm.org/D34958

llvm-svn: 307842