[AArch64] NFC: Generalize emitFrameOffset to support more than byte offsets.

Refactor emitFrameOffset to accept a StackOffset struct as its offset argument.
This method currently only supports byte offsets (MVT::i8) but will be extended
in a later patch to support scalable offsets (MVT::nxv1i8) as well.

Reviewers: thegameg, rovka, t.p.northover, efriedma, greened

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D61436

llvm-svn: 368049