AMDGPU: Refactor MIMG instruction TableGen using generic tables

Summary:
This allows us to access rich information about MIMG opcodes from C++ code.
Simplifying the mapping between equivalent opcodes of different data size
becomes quite natural.

This also flattens the MIMG-related class and multiclass hierarchy a little,
and collapses together some of the scaffolding for sample and gather4 opcodes.

Change-Id: I1a2549fdc1e881ff100e5393d2d87e73729a0ccd

Reviewers: arsenm, rampitec

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

Differential Revision: https://reviews.llvm.org/D48016

llvm-svn: 335227