[AArch64][SVE] Fix CFA calculation in presence of SVE objects.

The CFA is calculated as (SP/FP + offset), but when there are
SVE objects on the stack the SP offset is partly scalable and
should instead be expressed as the DWARF expression:

     SP + offset + scalable_offset * VG

where VG is the Vector Granule register, containing the
number of 64bits 'granules' in a scalable vector.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D84043