AMDGPU: Remove redundant MIMG instruction variants

Summary:
For sample and gather ops, we can accurately determine the set of
vaddr-size instruction variants that are required. This reduces
the size of instruction tables by ~5%.

The number of machine instruction opcodes is reduced from 10002
to 9476.

Change-Id: Ie7fc65d3657b762c7816017fe70b2e9bec644a8a

Reviewers: arsenm, rampitec

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

Differential Revision: https://reviews.llvm.org/D48168

llvm-svn: 335232