[AMDGPU] Rename AMDGPUCFGStructurizer to R600MachineCFGStructurizer

Previously the name of the class (AMDGPUCFGStructurizer) did not
match the name of the file (AMDILCFGStructurizer).

Standardize on the name R600MachineCFGStructurizer by analogy with
AMDGPUMachineCFGStructurizer.

Differential Revision: https://reviews.llvm.org/D120128