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
1. Splitted out some parts of R600 target to separate modules/headers.
2. Reduced some include lists in headers.
3. Found and fixed issue with override `GCNTargetMachine::getSubtargetImpl()`
and `R600TargetMachine::getSubtargetImpl()` had different return value type
than base class.
4. Minor forward declarations cleanup.
Reviewed By: foad
Differential Revision: https://reviews.llvm.org/D108596
