[AMDGPU] Fix typo in SIInstrInfo::memOpsHaveSameBasePtr

Summary:
The typo has been present since memOpsHaveSameBasePtr was introduced in
r313208.

It caused SIInstrInfo::shouldClusterMemOps to cluster more mem ops than
it was supposed to.

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71616