[AMDGPU] Handle frame index base operands in memOpsHaveSameBasePtr

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

Reviewers: arsenm, rampitec

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73454