[llvm] [AMDGPU] Rework getDivNumBits API (PR #119768)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> should have passed in `BitWidth - 32` instead of `32`

I'd prefer to just do this as a separate functionality fix without mixing in the API change 

https://github.com/llvm/llvm-project/pull/119768