AMDGPU: Remove old-style image intrinsics

Summary:
This also removes the need for atomic pseudo instructions, since
we select the correct encoding directly in SITargetLowering::lowerImage
for dimension-aware image intrinsics.

Mesa uses dimension-aware image intrinsics since
commit a9a7993441.

Change-Id: I7473d20009476a4ed6d919cae4e6dca9ff42e77a

Reviewers: arsenm, rampitec, mareko, tpr, b-sumner

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

Differential Revision: https://reviews.llvm.org/D48167

llvm-svn: 335231