The expansion of a SCEVUnknown is trivial (it's just the wrapped value).
If we try to reuse an existing value it might be a more complex
expression that simplifies to the SCEVUnknown.
This is inspired by https://github.com/llvm/llvm-project/issues/114879,
because SCEVExpander replacing a constant with a phi node is just silly.
(I don't consider this a fix for that issue though.)
6dc23b7009