[mlir][shape] Add `shape.get_extent`.

Summary:
This op extracts an extent from a shape.

This also is the first op which constant folds to shape.const_size,
which revealed that shape.const_size needs a folder (ConstantLike ops
seem to always need folders for the constant folding infra to work).

Differential Revision: https://reviews.llvm.org/D80394