AMDGPU/SILoadStoreOptimizer: fix a likely bug introduced recently

Summary:
We should check for same instruction class before checking whether they
have the same base address, else we might iterate out of bounds of a
MachineInstr operands list. The InstClass check is also cheaper.

This was introduced in SVN r373630.

Reviewers: tstellar

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68690