[AMDGPU] Refactor ds_read/ds_write related select code for better readability.

Part of the code related to ds_read/ds_write ISel is refactored, and the
corresponding comment is re-written for better readability, which would help
while implementing any future ds_read/ds_write ISel related modifications.

Reviewed By: rampitec

Differential Revision: https://reviews.llvm.org/D100300