[mlir] Provide CustomOpAsmParser::parseOptionalOperand

[ftynse]

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.

[rriddle]

Are there any existing users that can take advantage of this?

[ftynse]

In D76779#1941937, @rriddle wrote:

Are there any existing users that can take advantage of this?

Yes, but in a private repo.

[mehdi_amini]

Could you add something in the test dialect to have upstream coverage?

mlir/include/mlir/IR/OpImplementation.h

struct OperandType {
     llvm::SMLoc location; // Location of the token.
     StringRef name;       // Value name, e.g. %42 or %abc
     unsigned number;      // Number, e.g. 12 for an operand like %xyz#12
   };
 
   /// 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 {
     /// Zero or more operands with no delimiters.
     None,
     /// Parens surrounding zero or more operands.
     Paren,
     /// Square brackets surrounding zero or more operands.

mlir/lib/Parser/Parser.cpp

ParseResult parseOperand(OperandType &result) override {
     OperationParser::SSAUseInfo useInfo;
     if (parser.parseSSAUse(useInfo))
       return failure();
 
     result = {useInfo.loc, useInfo.name, useInfo.number};
     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,
                                int requiredOperandCount = -1,
                                Delimiter delimiter = Delimiter::None) override {
     return parseOperandOrRegionArgList(result, /*isOperandList=*/true,
                                        requiredOperandCount, delimiter);
   }