[AMDGPU] Handle multiple base operands in shouldClusterMemOps

Summary:
This is in preparation for getMemOperandsWithOffset returning more base
operands.

Depends on D73454.

Reviewers: arsenm, rampitec

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73455