AMDGPU: Remove AMDGPUMCInstLower.h

Summary:
The AMDGPUMCInstLower class is not used outside AMDGPUMCInstLower.cpp,
so we don't need a header file.

Reviewers: arsenm, nhaehnle

Reviewed By: arsenm

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

Differential Revision: https://reviews.llvm.org/D47264

llvm-svn: 333254