Make broadcast recursion in `flatten` structural (#29816)
* Make broadcast recursion in `flatten` structural
The inference enhancements in #29294 work quite well to prevent limiting
on many kinds of code. However, targetting TPUs, one code pattern it
struggeled with was a fairly large broadcast fusion in Flux:
λ.(reshape(γ, affine_shape...) .* ((x .- μ) ./ σ) .+ reshape(β, affine_shape...))
The reason #29294 is because the make_makeargs function used by the
implementation of Broadcast.flatten (which the TPU backend uses) had
a non-decreasing first argument (passing the return value of a previous
invocation of make_makeargs back in as the first argument). However,
that's not a fundamental limitation of the operation, but rather an
implementation choice. This PR switches that function's recursion pattern
to be purely structural, allowing inference to infer through it (with
the changes in #29294). As a result, ResNet50 infers properly.
* Comment spelling fix
Co-Authored-By: mbauman <mbauman@gmail.com>
