[AMDGPU/MemOpsCluster] Compute `width` for `MIMG` instruction class.

Summary:
`width` computation is missing for newly added `MIMG`
instruction class. Add it.

Reviewers: foad, rampitec, arsenm

Reviewed By: foad

Subscribers: MatzeB, javed.absar, kzhuravl, jvesely, wdng, nhaehnle, yaxunl, dstuttard, tpr, t-tye, hiraditya, kerbowa, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D81649