AMDGPU/SI: Add a MachineMemOperand to MIMG instructions

Summary:
Without a MachineMemOperand, the scheduler was assuming MIMG instructions
were ordered memory references, so no loads or stores could be reordered
across them.

Reviewers: arsenm

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

Differential Revision: https://reviews.llvm.org/D27536

llvm-svn: 290179