[llvm] [AMDGPU] AMDGPULateCodeGenPrepare Legacy PM: replace `setPreservesAll()` with `setPreservesCFG()` (PR #148167)

[Jim M. R. Teichgräber via llvm-commits]

J-MR-T wrote:

Ah, good point, I didn't consider that the description is included as well. Given that the notes aren't too long, always showing them shouldn't matter too much. Thanks!

https://github.com/llvm/llvm-project/pull/148167