[PATCH] AArch64: Add lowering of llvm.frameaddress and llvm.returnaddress

[Tim Northover]

Hi Weiming,

> Attached patch fixes PR17690: http://llvm.org/bugs/show_bug.cgi?id=17690 by
> add lowering of FRAMEADDR and RETURNADDR.

I think the patch looks fine (I was confused for a bit over just what
MachineFunction::addLiveIn does, but it seems sufficient for the job
at hand). Thanks for implementing that!

Cheers.

Tim.