[AArch64] Model ZA array using inaccessible memory (#132058)

This patch changes how ZA array is modelled at LLVM-IR level. Currently
accesses to ZA are represented at LLVM-IR level as memory reads and
writes and at instruction level as unmodeled side-effects. This patch
changes that and models them as purely Inaccessible memory accesses
without any unmodeled side-effects.

Author: Lukacma