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

[Fabian Ritter via llvm-commits]

ritter-x2a wrote:

@jayfoad you haven't marked this PR as approved; should I wait for other reviewers to chime in or are we good to go?

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