[AMDGPU][SILoadStoreOptimizer] NFC: Refactor code

Summary:
This patch fixes a potential aliasing problem in InstClassEnum,
where local values were mixed with machine opcodes.

Introducing InstSubclass will keep them separate and help extending
InstClassEnum with other instruction types (e.g. MIMG) in the future.

This patch also makes getSubRegIdxs() more concise.

Reviewers: nhaehnle, arsenm, tstellar

Reviewed By: arsenm

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68384

llvm-svn: 373699