[mlir][Shape][NFC] Migrate shape dialect to the new fold API

See https://discourse.llvm.org/t/psa-new-improved-fold-method-signature-has-landed-please-update-your-downstream-projects/67618 for context

Changes are mostly mechanical in nature. The code nevertheless became more expressive in a lot of places thanks to the use of the new getters!

Differential Revision: https://reviews.llvm.org/D141501