[AMDGPU][LowerModuleLDS] Refactor partially lowered module detection (#85793)

Refactor the logic that checks if a module contains mixed
absolute/non-lowered LDS GVs.

The check now happens latter when the "worklists" are formed. This is
because in some cases (OpenMP) we can have non-lowered GVs in a lowered
module, and this is normal because those GVs are just unused and removed
from the list at some point before the end of `getUsesOfLDSByFunction`.

Doing the check later ensures that if a mixed module is spotted, then
it's a _real_ mixed module that needs rejection, not a module containing
an intentionally ignored GV.