Improve precision of nfields_tfunc on `Union` (#37497)
`nfields_tfunc(Tuple{Int, Union{Int, Float64}})` gave `Const(2)`,
but `nfields_tfunc(Union{Tuple{Int, Float64}, Tuple{Int, Int}})` gave
`Int`. In general, it's not great for the tfuncs to give different
answers when given `==` types, because other parts of the system
will happily substitute such types without changes to the user code,
which can change the inference result and lead to instability.
