[Attributor] Avoid the type check in getCalledFunction

We now consistently use `CallBase::getCalledOperand` rather than
`getCalledFunction`, as we do not want the type checked performed by the
latter. This exposed various missing checks to handle mismatches
properly, but it is good to have them explicit now.

In a follow up we might want to flag certain calls as UB, but for now,
we allow everything to cut down on unexpected differences.