[AMDGPU] Replace non-kernel function uses of LDS globals by pointers.

The main motivation behind pointer replacement of LDS use within non-kernel
functions is - to *avoid* subsequent LDS lowering pass from directly packing
LDS (assume large LDS) into a struct type which would otherwise cause allocating
huge memory for struct instance within every kernel.

Reviewed By: rampitec

Differential Revision: https://reviews.llvm.org/D103225