De-virtualize mnemonicIsValid and remove from the base class. It's not called by any common code.

llvm-svn: 256544
diff --git a/llvm/utils/TableGen/AsmMatcherEmitter.cpp b/llvm/utils/TableGen/AsmMatcherEmitter.cpp
index 988e671..1ac2c303 100644
--- a/llvm/utils/TableGen/AsmMatcherEmitter.cpp
+++ b/llvm/utils/TableGen/AsmMatcherEmitter.cpp
@@ -2700,7 +2700,7 @@
      << "&Operands);\n";
   OS << "  void convertToMapAndConstraints(unsigned Kind,\n                ";
   OS << "           const OperandVector &Operands) override;\n";
-  OS << "  bool mnemonicIsValid(StringRef Mnemonic, unsigned VariantID) override;\n";
+  OS << "  bool mnemonicIsValid(StringRef Mnemonic, unsigned VariantID);\n";
   OS << "  unsigned MatchInstructionImpl(const OperandVector &Operands,\n"
      << "                                MCInst &Inst,\n"
      << "                                uint64_t &ErrorInfo,"