[MC] Delete MCFragment::isDummy. NFC

isa<...>, dyn_cast<...> and cast<...> are used by other fragments.
Don't make MCDummyFragment special.
diff --git a/llvm/lib/MC/MCFragment.cpp b/llvm/lib/MC/MCFragment.cpp
index 5192948..f6b64c8 100644
--- a/llvm/lib/MC/MCFragment.cpp
+++ b/llvm/lib/MC/MCFragment.cpp
@@ -236,7 +236,7 @@
                        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);
 }