[AMDGPU] getMemOperandsWithOffset: add resource operand for BUF instructions

Summary:
This prevents unwanted clustering of BUF instructions with the same
vaddr but different resource descriptors.

Reviewers: rampitec, arsenm, nhaehnle

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73867