Initial AArch64 SLH implementation.

This is an initial implementation for Speculative Load Hardening for
AArch64. It builds on top of the recently introduced
AArch64SpeculationHardening pass.
This doesn't implement (yet) some of the optimizations implemented for
the X86SpeculativeLoadHardening pass. I thought introducing the
optimizations incrementally in follow-up patches should make this easier
to review.

Differential Revision: https://reviews.llvm.org/D55929

llvm-svn: 350729