[mlir] Move tblgen code generation to use functional forms of cast/isa

Summary:
The method forms are deprecated. This updates the rest of the tblgen
uses of methods where a function call is available.

Context:
- https://mlir.llvm.org/deprecation/ at "Use the free function variants for dyn_cast/cast/isa/…"
- Original discussion at https://discourse.llvm.org/t/preferred-casting-style-going-forward/68443

Reviewers: rriddle