AMDGPU/SILoadStoreOptimizer: Make some functions const

Reviewers: arsenm, pendingchaos, rampitec

Reviewed By: rampitec

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65316

llvm-svn: 367517