[AMDGPU][SILoadStoreOptimizer] Merge TBUFFER loads/stores

Summary: Extend SILoadStoreOptimizer to merge tbuffer loads and stores.

Reviewers: nhaehnle

Reviewed By: nhaehnle

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69794