[mlir] Add additional Canonicalization of shape.cstr_broadcastable.

Summary:
Added canonicalization and folding was:
- Folding when either input is an attribute indicating a scalar input
which can always be broadcasted.
- Canonicalization where it can be determined that either input shape is
a scalar.
- Canonicalization where the partially specified input shapes can be
proven to be broadcastable always.

Differential Revision: https://reviews.llvm.org/D83194