AMDGPU/SILoadStoreOptimillzer: Refactor CombineInfo struct

Summary:
Modify CombineInfo to only store information about a single instruction.
This is a little easier to work with and removes a lot of duplicate
initialization code.

Reviewers: arsenm, nhaehnle

Reviewed By: arsenm, nhaehnle

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71045