AMDGPU: Pass AMDGPUSampleVariant to MIMG_{Sampler,Gather}(_WQM)

Summary:
This will allows us to provide rich metadata about the instructions
in tables that are accessible by custom C++ code.

Change-Id: Id9305a26304ab6a6cceb6c65c8cd49141cc0101d

Reviewers: arsenm, rampitec

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

Differential Revision: https://reviews.llvm.org/D48011

llvm-svn: 335224