[AArch64] Prepare for split ZPR and PPR area allocation (NFCI)

This patch attempts to refactor AArch64FrameLowering to allow the size
of the ZPR and PPR areas to be calculated separately. This will be used
by a subsequent patch to support allocating ZPRs and PPRs to separate
areas. This patch should be an NFC and is split out to make later
functional changes easier to spot.

Author: MacDue