[lldb][AArch64] Check SIMD save/restore in SVE SIMD test

While doing some refactoring I forgot to carry over the copying in of
SIMD data in normal mode, but no tests failed.

Turns out, it's very easy for us to get the restore wrong because
even if you forget the memcopy, setting the buffer to valid may
just read the data you had before the expression evaluation.

So I've extended the SVE SIMD testing (which includes the plain SIMD mode)
to check expression save/restore. This is the only test that fails
if you forget to do `m_fpu_is_valid = true` so I take from that, that
prior to this it wasn't tested at all.

As a bonus, we now have coverage of the same thing for SVE and SSVE modes.

Reviewed By: omjavaid

Differential Revision: https://reviews.llvm.org/D157000