[mlir-tblgen] Emit named operand indices

[Meinersbur]

An operation's operands are defined by the arguments field in the tablegen definition. mlir-tblgen generates accessors for them: getXYZ() and setXYZ(...) to set an operation's operands without knowing the operand's index, but it does not expose the operand index itself. Yet some use cases requires knowing the operand index that is now covered by just getters and setters. For instance:

• Given an mlir::OpOperand, find out whether it is a specific argument (from the arguments field in the .td file)
• For operation with variable number of operands (variadic, AttrSizedOperandSegments), get the value to pass to getODSOperands or getODSOperandIndexAndLength.

Emitting a named constant avoids hardcoding the index in the source as magic constant, such as 2 in

llvm-project/mlir/lib/Dialect/LLVMIR/IR/NVVMDialect.cpp

Line 244 in d56c06e

getODSOperands(2).getTypes().front(), /*isAccumulator=*/true);

Extracted out of #144785