[Mips][CodeGen] Remove MachineFunction::setSubtarget. Change Mips to just copy the subtarget from the MachineFunction instead of recalculating it.

[craig.topper]

Summary

The MachineFunction should have been created with the correct subtarget. As
long as there is no way to change it, MipsTargetMachine can just capture it
directly from the MachineFunction without calling getSubtargetImpl again.

While there, const correct the Subtarget pointer to avoid a const_cast.

I believe the Mips16Subtarget and NoMips16Subtarget members are never used, but
I'll leave there removal for a separate patch.

[atanasyan]

LGTM

llvm/include/llvm/CodeGen/MachineFunction.h

public:
   unsigned getFunctionNumber() const { return FunctionNumber; }
 
   /// getTarget - Return the target machine this machine code is compiled with
   const LLVMTargetMachine &getTarget() const { return Target; }
 
   /// getSubtarget - Return the subtarget for which this machine code is being
   /// compiled.
   const TargetSubtargetInfo &getSubtarget() const { return *STI; }
-  void setSubtarget(const TargetSubtargetInfo *ST) { STI = ST; }
 
   /// getSubtarget - This method returns a pointer to the specified type of
   /// TargetSubtargetInfo.  In debug builds, it verifies that the object being
   /// returned is of the correct type.
   template<typename STC> const STC &getSubtarget() const {
     return *static_cast<const STC *>(STI);
   }
 

llvm/lib/Target/Mips/MipsTargetMachine.h

 
 namespace llvm {
 
 class MipsTargetMachine : public LLVMTargetMachine {
   bool isLittle;
   std::unique_ptr<TargetLoweringObjectFile> TLOF;
   // Selected ABI
   MipsABIInfo ABI;
-  MipsSubtarget *Subtarget;
+  const MipsSubtarget *Subtarget;
   MipsSubtarget DefaultSubtarget;
   MipsSubtarget NoMips16Subtarget;
   MipsSubtarget Mips16Subtarget;
 
   mutable StringMap<std::unique_ptr<MipsSubtarget>> SubtargetMap;
 
 public:
   MipsTargetMachine(const Target &T, const Triple &TT, StringRef CPU,

llvm/lib/Target/Mips/MipsTargetMachine.cpp

I = llvm::make_unique<MipsSubtarget>(TargetTriple, CPU, FS, isLittle, *this,
                                          Options.StackAlignmentOverride);
   }
   return I.get();
 }
 
 void MipsTargetMachine::resetSubtarget(MachineFunction *MF) {
   LLVM_DEBUG(dbgs() << "resetSubtarget\n");
 
-  Subtarget = const_cast<MipsSubtarget *>(getSubtargetImpl(MF->getFunction()));
-  MF->setSubtarget(Subtarget);
+  Subtarget = &MF->getSubtarget<MipsSubtarget>();
 }
 
 namespace {
 
 /// Mips Code Generator Pass Configuration Options.
 class MipsPassConfig : public TargetPassConfig {
 public:
   MipsPassConfig(MipsTargetMachine &TM, PassManagerBase &PM)