Factor out resolveFrameOffsetReference (NFC).

Split AArch64FrameLowering::resolveFrameIndexReference in two parts
* Finding frame offset for the index.
* Finding base register and offset to that register.

The second part will be used to implement a virtual frame pointer in
armv8.5 MTE stack instrumentation lowering.

Reviewers: pcc, vitalybuka, hctim, ostannard

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64171

llvm-svn: 365958