Let the stringifier print the '?' for unimplemented operands.

Author: vext01

llvm/lib/YkIR/YkIRWriter.cpp

@@ -162,7 +162,6 @@ class YkIRWriter {
   // the unhandled LLVM operand in textual form.
   void serialiseUnimplementedOperand(Value *V) {
     OutStreamer.emitInt8(OperandKind::String);
-    OutStreamer.emitInt8('?');
     serialiseString(toString(V));
   }