[AMDGPU] Add intrinsics for tbuffer load and store

[AMDGPU] Add intrinsics for tbuffer load and store

Intrinsic already existed for llvm.SI.tbuffer.store

Needed tbuffer.load and also re-implementing the intrinsic as llvm.amdgcn.tbuffer.*

Added CodeGen tests for the 2 new variants added.
Left the original llvm.SI.tbuffer.store implementation to avoid issues with existing code

Subscribers: arsenm, kzhuravl, wdng, nhaehnle, yaxunl, tony-tye, tpr

Differential Revision: https://reviews.llvm.org/D30687

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@306031 91177308-0d34-0410-b5e6-96231b3b80d8