(Reland) [mlir] Switch segment size attributes to DenseI32ArrayAttr

This reland includes changes to the Python bindings.

Switch variadic operand and result segment size attributes to use the
dense i32 array. Dense integer arrays were introduced primarily to
represent index lists. They are a better fit for segment sizes than
dense elements attrs.

Depends on D131801

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D131803