[ValueTracking] Loads with !dereferenceable metadata cannot be undef/poison

A load with !dereferenceable or !dereferenceable_or_null metadata
must return a well-defined (non-undef/poison) value. Effectively
they imply !noundef. This is the same as we do for the
dereferenceable(N) attribute.

This should fix https://github.com/llvm/llvm-project/issues/55672,
or at least the specific case discussed there.

Differential Revision: https://reviews.llvm.org/D126296