[InstCombine][AMDGPU] Handle more buffer intrinsics

Summary:
Include the following intrinsics in the InsctCombine
simplification:

* amdgcn_raw_buffer_load
* amdgcn_raw_buffer_load_format
* amdgcn_struct_buffer_load
* amdgcn_struct_buffer_load_format

Change-Id: I14deceff74bcb21179baf6aa6e94bf39e7d63d5d

Reviewers: arsenm

Reviewed By: arsenm

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

Differential Revision: https://reviews.llvm.org/D55882

llvm-svn: 349735