AMDGPU: Merge BUFFER_LOAD_DWORD_OFFSET into x2, x4

Summary: Only 3 (out of 48486) shaders are affected.

Reviewers: arsenm, nhaehnle

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

Differential Revision: https://reviews.llvm.org/D38951

llvm-svn: 317753