[MLIR] NFC: Change dyn_cast to isa to prevent warning (#81899)

This is to avoid warnings when invoked from the flang documentation
generation build.

The warning can be seen in the CI
(https://lab.llvm.org/buildbot/#/builders/89/builds/57451).
```
/home/buildbot/as-worker-4/publish-sphinx-docs/llvm-project/mlir/tools/mlir-tblgen/OpDefinitionsGen.cpp:2956:38: warning: unused variable ‘operand’ [-Wunused-variable]
```