[Attributor] Keep complex select and PHI instructions in AAPotentialValues

If a select or PHI instruction cannot be simplified to a single value it
is often beneficial to keep the value intact rather than looking at all
the operands. The reason is that various users require a single value
and consequently can deal with the select or PHI but not multiple
operands. Recursive calls of `Attributor::getAssumedSimplifiedValues`
will be required to take such select and PHI instructions apart.