Replace dyn_cast with isa in places that weren't using the returned value for more than a boolean check. NFC.

llvm-svn: 253441
diff --git a/llvm/lib/DebugInfo/DWARF/DWARFContext.cpp b/llvm/lib/DebugInfo/DWARF/DWARFContext.cpp
index 18c4e8e..2165d35 100644
--- a/llvm/lib/DebugInfo/DWARF/DWARFContext.cpp
+++ b/llvm/lib/DebugInfo/DWARF/DWARFContext.cpp
@@ -704,7 +704,7 @@
     // relocation point already factors in the section address
     // (actually applying the relocations will produce wrong results
     // as the section address will be added twice).
-    if (!L && dyn_cast<MachOObjectFile>(&Obj))
+    if (!L && isa<MachOObjectFile>(&Obj))
       continue;
 
     RelSecName = RelSecName.substr(