[llvm] [AMDGPU] Add flag to prevent reruns of LowerModuleLDS (PR #129520)

[Shilei Tian via llvm-commits]

https://github.com/shiltian approved this pull request.

I think ideally we want to use phase argument but since we don't have one for module splitting at the moment and it's also not clear how beneficial it is, so using a module flag looks fine.

https://github.com/llvm/llvm-project/pull/129520