[ARM][Debug Info] Restore emitting of .cfi_def_cfa_offset for functions without stack frame

Summary: Looks like new code from [[ http://reviews.llvm.org/rL222057 | rL222057 ]] doesn't account for early `return` in `ARMFrameLowering::emitPrologue`, which leads to loosing `.cfi_def_cfa_offset` directive for functions without stack frame.

Reviewers: echristo, rengolin, asl, t.p.northover

Reviewed By: t.p.northover

Subscribers: llvm-commits, rengolin, aemerson

Differential Revision: http://reviews.llvm.org/D8606

llvm-svn: 234399