[ARM] Custom machine-scheduler. NFCI.

[javed.absar]

Summary

This patch creates a customised machine-scheduler for ARM targets, so that subsequently DAG mutations etc
can be added.

[fhahn]

LGTM. This will allow us to add DAG mutations for CPUs using the MachineScheduler going forward.

lib/Target/ARM/ARMTargetMachine.cpp

 #include "llvm/CodeGen/GlobalISel/IRTranslator.h"
 #include "llvm/CodeGen/GlobalISel/InstructionSelect.h"
 #include "llvm/CodeGen/GlobalISel/InstructionSelector.h"
 #include "llvm/CodeGen/GlobalISel/Legalizer.h"
 #include "llvm/CodeGen/GlobalISel/LegalizerInfo.h"
 #include "llvm/CodeGen/GlobalISel/RegBankSelect.h"
 #include "llvm/CodeGen/GlobalISel/RegisterBankInfo.h"
 #include "llvm/CodeGen/MachineFunction.h"
+#include "llvm/CodeGen/MachineScheduler.h"
 #include "llvm/CodeGen/Passes.h"
 #include "llvm/CodeGen/TargetPassConfig.h"
 #include "llvm/IR/Attributes.h"
 #include "llvm/IR/DataLayout.h"
 #include "llvm/IR/Function.h"
 #include "llvm/Pass.h"
 #include "llvm/Support/CodeGen.h"
 #include "llvm/Support/CommandLine.h"

 public:
   ARMPassConfig(ARMBaseTargetMachine &TM, PassManagerBase &PM)
     : TargetPassConfig(TM, PM) {}
 
   ARMBaseTargetMachine &getARMTargetMachine() const {
     return getTM<ARMBaseTargetMachine>();
   }
 
+  ScheduleDAGInstrs *
+  createMachineScheduler(MachineSchedContext *C) const override {
+    ScheduleDAGMILive *DAG = createGenericSchedLive(C);
+    // add DAG Mutations here.
+    return DAG;
+  }
+
+  ScheduleDAGInstrs *
+  createPostMachineScheduler(MachineSchedContext *C) const override {
+    ScheduleDAGMI *DAG = createGenericSchedPostRA(C);
+    // add DAG Mutations here.
+    return DAG;
+  }
+
   void addIRPasses() override;
   bool addPreISel() override;
   bool addInstSelector() override;
 #ifdef LLVM_BUILD_GLOBAL_ISEL
   bool addIRTranslator() override;
   bool addLegalizeMachineIR() override;
   bool addRegBankSelect() override;
   bool addGlobalInstructionSelect() override;