[AArch64] Stackframe accesses to SVE objects.

Materialize accesses to SVE frame objects from SP or FP, whichever is
available and beneficial.

This patch still assumes the objects are pre-allocated. The automatic
layout of SVE objects within the stackframe will be added in a separate
patch.

Reviewers: greened, cameron.mcinally, efriedma, rengolin, thegameg, rovka

Reviewed By: cameron.mcinally

Differential Revision: https://reviews.llvm.org/D67749

llvm-svn: 374772