[amdgpu][nfc] Replace ad hoc LDS frame recalculation with absolute_symbol MD

Post ISel, LDS variables are absolute values. Representing them as
such is simpler than the frame recalculation currently used to build assembler
tables from their addresses.

This is a precursor to lowering dynamic/external LDS accesses from non-kernel
functions.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D144221