Fix use-after-free bug in AffectedValueCallbackVH::allUsesReplacedWith

When transferring affected values in the cache from an old value,
identified by the value of the current callback, to the specified new
value we might need to insert a new entry into the DenseMap which
constitutes the cache. Doing so might delete the current callback
object. Move the copying logic into a new function, a member of the
assumption cache itself, so that we don't run into UB should the
callback handle itself be removed mid-copy.

Differential Revision: https://reviews.llvm.org/D28749