[AMDGPU/MemOpsCluster] Code clean-up around accessing of memory operand width

Summary:
Clean-up the width computing logic given a memory operand, and re-arrange code to avoid
code duplication.

Reviewers: foad, rampitec, arsenm, vpykhtin, javedabsar

Reviewed By: foad

Subscribers: kzhuravl, jvesely, wdng, nhaehnle, yaxunl, dstuttard, tpr, t-tye, hiraditya, kerbowa, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80946