[ARM][MVE] Fix location of optimized gather addresses

Fix for the address optimization for gathers and scatters which would in
some complex cases push out instructions not to the vector loop preheader,
but to other locations as well which lead to a scrambled order and the
compilation failing.
This patch ensures that said instructions are always pushed to the end
of the vector loop preheader.

Differential Revision: https://reviews.llvm.org/D78293