[amdgpu] Expand all ConstantExpr users of LDS variables in instructions

Bug noted in D112717 can be sidestepped with this change.

Expanding all ConstantExpr involved with LDS up front makes the variable specialisation simpler. Excludes ConstantExpr that don't access LDS to avoid disturbing codegen elsewhere.

Reviewed By: rampitec

Differential Revision: https://reviews.llvm.org/D133422