AMDGPU/SILoadStoreOptimizer: Add helper functions for working with CombineInfo

Summary:
This is a refactoring that will make future improvements to this pass easier.
This change should not change the behavior of the pass.

Reviewers: arsenm, pendingchaos, rampitec, nhaehnle, vpykhtin

Reviewed By: nhaehnle, vpykhtin

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65496

llvm-svn: 373366