[AMDGPU] Simplify DS and SM cases in getMemOperandsWithOffset

Summary:
This removes a couple of unnecessary isReg checks, now that
memOpsHaveSameBasePtr can handle FI operands, but is otherwise NFC.

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/D73485