[clang] [CIR] Use getDefiningOp<OpTy>() instead of dyn_cast<OpTy>(getDefiningOp()) (NFC) (PR #151217)

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graphite-app[bot] wrote:

### Merge activity

* **Aug 1, 7:09 PM UTC**: This pull request can not be added to the [Graphite merge queue](https://app.graphite.dev/merges?org=llvm&repo=llvm-project). Please try rebasing and resubmitting to merge when ready. 


https://github.com/llvm/llvm-project/pull/151217