[amdgpu][nfc] Remove dead code associated with LDS lowering

Pass disabled since approximately D104962 for miscompiling openmp

The functions under ReplaceConstant miscompile phis as noted in D112717 and
have no users in tree other than the disabled pass. It seems likely it has no
users out of tree.

Deletes the test cases associated with the disabled pass as well.

Reviewed By: rampitec

Differential Revision: https://reviews.llvm.org/D147586