1) Overloaded (instruction-based) method is a wrapper around the current (opcode-based) method.
2) This patch also changes a few callsites (VectorCombine.cpp,
SLPVectorizer.cpp, CodeGenPrepare.cpp) to call the overloaded method.
3) This is a split of D128302.
Differential Revision: https://reviews.llvm.org/D131114
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
