[llvm] [AMDGPUPromoteAlloca][NFC] Avoid unnecessary APInt/int64_t conversions (PR #157864)

[Fabian Ritter via llvm-commits]

ritter-x2a wrote:

### Merge activity

* **Sep 12, 7:49 AM UTC**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/157864).


https://github.com/llvm/llvm-project/pull/157864