Callback-based loop construction, with loop bodies being constructed during the
construction of the parent op using a function, is now fully supported by the
core infrastructure. This provides almost the same level of brevity as EDSC
LoopBuilder at less than 30% infrastructural code cost. Functional equivalents
compatible with EDSC ScopedContext are implemented on top of the main builders.
LoopBuilder and related functionality has been deprecated, remove it.
Differential Revision: https://reviews.llvm.org/D81874
5c5ec96bd2