Enables llvm dyn_cast for Assemblers.

src/IceAssemblerX8664.h

 //===- subzero/src/IceAssemblerX8664.h - Assembler for x86-64 -*- C++ -*---===//
 //
 //                        The Subzero Code Generator
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 //
 // This file implements the Assembler class for x86-64.
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef SUBZERO_SRC_ICEASSEMBLERX8664_H
 #define SUBZERO_SRC_ICEASSEMBLERX8664_H
 
 #include "IceAssembler.h"
 #include "IceDefs.h"
 
 namespace Ice {
 namespace X8664 {
 
 class AssemblerX8664 final : public Assembler {
   AssemblerX8664(const AssemblerX8664 &) = delete;
   AssemblerX8664 &operator=(const AssemblerX8664 &) = delete;
 
 public:
-  explicit AssemblerX8664(bool use_far_branches = false) : Assembler() {
+  explicit AssemblerX8664(bool use_far_branches = false)
+      : Assembler(Asm_X8664) {
     assert(!use_far_branches);
     (void)use_far_branches;
     llvm::report_fatal_error("Not yet implemented");
   }
 
   ~AssemblerX8664() override = default;
 
   void alignFunction() override;
   void padWithNop(intptr_t Padding) override;
   SizeT getBundleAlignLog2Bytes() const override;
   const char *getNonExecPadDirective() const override;
   llvm::ArrayRef<uint8_t> getNonExecBundlePadding() const override;
   void bindCfgNodeLabel(SizeT NodeNumber) override;
   bool fixupIsPCRel(FixupKind Kind) const override;
+
+  static bool classof(const Assembler *Asm) {
+    return Asm->getKind() == Asm_X8664;
+  }
 };
 
 } // end of namespace X8664
 } // end of namespace Ice
 
 #endif // SUBZERO_SRC_ICEASSEMBLERX8664_H