[AMDGPU] Let LowerModuleLDS run twice on the same module (#81729)

If all variables in the module are absolute, this means we're running
the pass again on an already lowered module, and that works.
If none of them are absolute, lowering can proceed as usual.
Only diagnose cases where we have a mix of absolute/non-absolute GVs,
which means we added LDS GVs after lowering, which is broken.

See #81491
Split from #75333