[Attributor] Replace AAReturnedValues with AAPotentialValuesReturned

The very first AA, at least the first one in order, is not necessary
anymore. `AAReturnedValues` was from a different time; one might say, a
simpler time.

It was rewriten once to use `Attribute::getAssumedSimplifiedValues`,
which is what the replacement, `AAPotentialValuesReturned`, does too.
To match the old behavior we needed to avoid the helper
`AAReturnedFromReturnedValues` and iterate the return instructions
explicitly, however, it is still less complexity than it was before.
`AAReturnedFromReturnedValues` and `getAssumedSimplifiedValues` now
allow users to stop at PHI and select nodes or to ignore those and look
through. `AANoFPClass` will stop at select and phi nodes to read the
fast math flags.

Fixes: https://github.com/llvm/llvm-project/issues/63404

Differential Revision: https://reviews.llvm.org/D154917