message: Add the LLVM IR unreachable instruction to the LLVMIR dialect.
  Closes
  COPYBARA_INTEGRATE_REVIEW=https://github.com/tensorflow/mlir/pull/64 from schweitzpgi:unreachable-op
diff: @@ -300,6 +300,11 @@ def LLVM_ReturnOp : LLVM_TerminatorOp<"return", []> {
  let parser = [{ return parseReturnOp(parser, result); }];
  let printer = [{ printReturnOp(p, *this); }];
  }
  +def LLVM_UnreachableOp : LLVM_TerminatorOp<"unreachable", []> {
  + string llvmBuilder = [{ builder.CreateUnreachable(); }];
  + let parser = [{ return success(); }];
  + let printer = [{ *p << getOperationName(); }];
  +}
  // Pseudo-operations (do not appear in LLVM IR but necessary for the dialect to
  // work correctly).
  