[Attributor] Avoid replacing return operands twice

As replacements will become more complex it is better to have a single
AA responsible for replacing a use. Before this patch AAValueSimplify*
and AAValueSimplifyReturned could both try to replace the returned
value. The latter was marginally better for the old pass manager
when a function was already carrying a `returned` attribute and when
the context of the return instruction was important. The second
shortcoming was resolved by looking for return attributes in the
AAValueSimplifyCallSiteReturned initialization. The old PM impact is
not concerning.

This is yet another step towards the removal of AAReturnedValues, the
very first AA we should now try to eliminate due to the overlapping
logic with value simplification.