[MC] Delete MCFragment::isDummy. NFC

isa<...>, dyn_cast<...> and cast<...> are used by other fragments.
Don't make MCDummyFragment special.

llvm/include/llvm/MC/MCFragment.h

@@ -106,9 +106,6 @@ public:
   /// this is false, but specific fragment types may set it to true.
   bool hasInstructions() const { return HasInstructions; }
 
-  /// Return true if given frgment has FT_Dummy type.
-  bool isDummy() const { return Kind == FT_Dummy; }
-
   void dump() const;
 };
 

llvm/lib/MC/MCFragment.cpp

@@ -236,7 +236,7 @@ MCFragment::MCFragment(FragmentType Kind, bool HasInstructions,
                        MCSection *Parent)
     : Kind(Kind), HasInstructions(HasInstructions), LayoutOrder(0),
       Parent(Parent), Atom(nullptr), Offset(~UINT64_C(0)) {
-  if (Parent && !isDummy())
+  if (Parent && !isa<MCDummyFragment>(*this))
     Parent->getFragmentList().push_back(this);
 }