BufferizeAnyLinalgOp fails because FillOp is not a LinalgGenericOp and it fails while reading operand sizes attribute.
FillOp abides by the structured op interface, what is annoying here is there is no adaptor for an interface.
Here is what I am currently iterating on, which does not use the conversion infrastructure: https://github.com/google/iree-llvm-sandbox/blob/main/runners/LinalgComprehensiveBufferizePass.cpp#L1043.
I guess this is fine in the meantime but I would be very cautious on depending on anything related to core linalg bufferization: it will likely break in a bunch of ways before it gets better (see e.g. https://llvm.discourse.group/t/properly-using-bufferization-related-passes/2913).