Don't allow isFP64bit for mips fast-isel at this time.

[rkotler]

Summary

We don't currently support isFP64bit at this time but it will be added very soon.
There are some differences and the current mips fast-isel will generate the wrong sequences
for that variant.

[rkotler]

Decision was made to solve this issue in another way.

lib/Target/Mips/MipsFastISel.cpp

explicit MipsFastISel(FunctionLoweringInfo &funcInfo,
         M(const_cast<Module &>(*funcInfo.Fn->getParent())),
         TM(funcInfo.MF->getTarget()),
         TII(*TM.getSubtargetImpl()->getInstrInfo()),
         TLI(*TM.getSubtargetImpl()->getTargetLowering()),
         Subtarget(&TM.getSubtarget<MipsSubtarget>()) {
     MFI = funcInfo.MF->getInfo<MipsFunctionInfo>();
     Context = &funcInfo.Fn->getContext();
     TargetSupported = ((Subtarget->getRelocationModel() == Reloc::PIC_) &&
+                       !Subtarget->isFP64bit() &&
                        ((Subtarget->hasMips32r2() || Subtarget->hasMips32()) &&
                         (Subtarget->isABI_O32())));
   }
 
   bool fastSelectInstruction(const Instruction *I) override;
   unsigned fastMaterializeConstant(const Constant *C) override;
 
   bool ComputeAddress(const Value *Obj, Address &Addr);