[AMDGPU] Refactor MIMG tables to better handle hardware variants

Add mimgopc object to represent the opcode allowing different
opcodes for different hardware variants.
This enables image_atomic_fcmpswap, image_atomic_fmin, and
image_atomic_fmax on GFX10

Reviewed By: foad, rampitec

Differential Revision: https://reviews.llvm.org/D96309