[MemCpyOpt] Use dereferenceable pointer helper

The call slot optimization has some home-grown code for checking
whether the destination is dereferenceable. Replace this with the
generic isDereferenceableAndAlignedPointer() helper.

I'm not checking alignment here, because that is currently handled
separately and may be an enforced alignment for allocas. The clean
way of integrating that part would probably be to accept a callback
in isDereferenceableAndAlignedPointer() for the actual isAligned check,
which would then have a chance to use an enforced alignment instead.

This allows the destination to be a GEP (among other things), though
the two open TODOs may prevent it from working in practice.

Differential Revision: https://reviews.llvm.org/D88805