[mlir] Add insert before/after to list-like constructs in C API

Blocks in a region and operations in a block are organized in a linked list.
The C API only provides functions to append or to insert elements at the
specified numeric position in the list. The latter is expensive since it
requires to traverse the list. Add insert before/after functionality with low
cost that relies on the iplist elements being convertible to iterators.

Reviewed By: stellaraccident

Differential Revision: https://reviews.llvm.org/D88148