[Attributor][FIX] Do not use assumed information for UB detection

The helper `Attributor::checkForAllReturnedValuesAndReturnInsts`
simplifies the returned value optimistically. In `AAUndefinedBehavior`
we cannot use such optimistic values when deducing UB. As a result, we
assumed UB for the return value of a function because we initially
(=optimistically) thought the function return is `undef`. While we later
adjusted this properly, the `AAUndefinedBehavior` was under the
impression the return value is "known" (=fix) and could never change.

To correct this we use `Attributor::checkForAllInstructions` and then
manually to perform simplification of the return value, only allowing
known values to be used. This actually matches the other UB deductions.

Fixes #53647