[AssumptionCache] Do not track llvm.assume calls (PR49043)

This fixes PR49043 by invalidating the handle on RAUW. This will work
fine assuming all existing RAUW users add the new assumption to the
cache. That means, if a new llvm.assume call replaces an old one, you
need to add the new one now as a RAUW is not enough anymore.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D96208