Thanks to a recent change that made ::build functions take an instance of
OpBuilder, it is now possible to build operations within a region attached to
the operaiton about to be created. Exercise this on loop::ForOp by taking a
callback that populates the loop body while the loop is being created.
Additionally, provide helper functions to build perfect nests of ForOps,
with support for iteration arguments. These functions provide the same
functionality as EDSC LoopNestBuilder with simpler implementation, without
relying on edsc::ScopedContext, and using OpBuilder in an unambiguous way.
Compatibility functions for EDSC are provided, but may be removed in the