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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@331490 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed