AMDGPU/SI: Remove assert from AMDGPUOpenCLImageTypeLowering pass

Summary:
Instead of asserting when the kernel metadata is different than we expect,
we should just skip lowering that function.  This fixes assertion
failures with OpenCL argument metadata from older LLVM releases.

Reviewers: arsenm

Subscribers: arsenm, llvm-commits

Differential Revision: http://reviews.llvm.org/D13356

llvm-svn: 249073