Make shape.is_broadcastable/shape.cstr_broadcastable nary

This corresponds with the previous work to make shape.broadcast nary.
Additionally, simplify the ConvertShapeConstraints pass. It now doesn't
lower an implicit shape.is_broadcastable. This is still the same in
combination with shape-to-standard when the 2 passes are used in either
order.

Differential Revision: https://reviews.llvm.org/D96401