887222e352
ArrayRef has a constructor that accepts std::nullopt. This constructor dates back to the days when we still had llvm::Optional. Since the use of std::nullopt outside the context of std::optional is kind of abuse and not intuitive to new comers, I would like to move away from the constructor and eventually remove it. This patch takes care of the mlir side of the migration, starting with straightforward places where I see ArrayRef or ValueRange nearby. Note that ValueRange has a constructor that forwards arguments to an ArrayRef constructor.
Multi-Level Intermediate Representation
See https://mlir.llvm.org/ for more information.