fix promote_op function (#29739)
This function had some weird and broken extraneous code,
using Core.Inference directly is not recommended,
but where we are going to do it anyways, we should at least do it correctly.
The `cumsum` code had a test to assert that `Real + Real` was convertable to `Real`.
This was hacked into the code poorly. It is now hacked in directly.
We can separately debate if this is a good idea,
I am simply preserving the existing behavior of the code
but implementing it correctly (by changing the behavior of the actual function,
instead of by convincing inference to return the incorrect answer).
