[llvm] [AMDGPU][SplitModule] Fix unintentional integer division (PR #117586)

[Fraser Cormack via llvm-commits]

frasercrmck wrote:

> Description should be more specific

Yep, you're right, sorry. I've just split it into two PRs since we can't merge individual commits in the same PR

https://github.com/llvm/llvm-project/pull/117586