inference: fix return_type_tfunc modeling of concrete functions (#51042)
The `aft` parameter is a value already, so we should be checking it in
the value domain, not the type domain like `tt`. That check happens to
already be done (somewhat unnecessarily) earlier in the function.
Fixes #40606
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Co-authored-by: Shuhei Kadowaki <40514306+aviatesk@users.noreply.github.com>