[AArch64] Move SLS later in pass pipeline

Currently, the SLS hardening pass is run before the machine outliner,
which means that the outliner creates new functions and calls which do
not have the SLS hardening applied.

The fix for this is to move the SLS passes to after the outliner, as has
recently been done for the return address signing pass.

This also avoids a bug where the SLS outliner emits code with
instructions after a return, which the outliner doesn't correctly
handle.

Reviewed By: kristof.beyls

Differential Revision: https://reviews.llvm.org/D158511