AMDGPU: Make getSubRegFromChannel a static member of AMDGPURegisterInfo

Summary:
This makes is possible to have R600RegisterInfo and SIRegisterInfo
not inherit from AMDGPURegisterInfo.

Reviewers: arsenm, nhaehnle

Reviewed By: arsenm

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

Differential Revision: https://reviews.llvm.org/D46280

llvm-svn: 331490