[amdgpu] Always, instead of mostly, remove unused LDS symbols

Currently LDS variables are removed by the lower module pass
if they have a use which is caught by the replace with struct control flow.
This makes tests brittle to changes to that control flow which induces
noise when trying to improve lowering. Some tests already check that
variables are removed, while others checked that they are not removed.

LDS variables are not (currently) externally accessible, and if that
changes the machinery which makes them externally accessible will look
like a use. This change therefore breaks no applications.

Reviewed By: rampitec

Differential Revision: https://reviews.llvm.org/D133028