[mlir] Provide CustomOpAsmParser::parseOptionalOperand

Summary:
Some operations have custom syntax where an operand is always followed by a
specific token of streams if the operand is present. Parsing such operations
requires the ability to optionally parse an operand. Provide a relevant
function in the custom Op parser.

Differential Revision: https://reviews.llvm.org/D76779

mlir/include/mlir/IR/OpImplementation.h

@@ -452,6 +452,9 @@ public:
   /// Parse a single operand.
   virtual ParseResult parseOperand(OperandType &result) = 0;
 
+  /// Parse a single operand if present.
+  virtual OptionalParseResult parseOptionalOperand(OperandType &result) = 0;
+
   /// These are the supported delimiters around operand lists and region
   /// argument lists, used by parseOperandList and parseRegionArgumentList.
   enum class Delimiter {

mlir/lib/Parser/Parser.cpp

@@ -4253,6 +4253,13 @@ public:
     return success();
   }
 
+  /// Parse a single operand if present.
+  OptionalParseResult parseOptionalOperand(OperandType &result) override {
+    if (parser.getToken().is(Token::percent_identifier))
+      return parseOperand(result);
+    return llvm::None;
+  }
+
   /// Parse zero or more SSA comma-separated operand references with a specified
   /// surrounding delimiter, and an optional required operand count.
   ParseResult parseOperandList(SmallVectorImpl<OperandType> &result,