[AMDGPU] Optimize image_[load|store]_mip

Summary:
Replace image_load_mip/image_store_mip
with image_load/image_store if lod is 0.

Reviewers: arsenm, nhaehnle

Reviewed By: arsenm

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63073

llvm-svn: 362957