[AMDGPU] Simplify SIRegisterInfo::getRegSplitParts

Summary:
Use more logic and fewer tables. This reduces the line count and
reduces the effort required to introduce more register classes of
different sizes in future.

Reviewers: arsenm, rampitec, nhaehnle

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78351