[mlir][SCF] Fix loop pipelining unable to handle ops with regions

This change allows the SCF LoopPipelining transform to handle ops with
nested regions within the pipelined `scf.for` body. The op and nested
regions are treated as a single unit from the transform's perspective.
This change also makes explicit the requirement that only ops whose
parent Block is the loop body Block are allowed to be scheduled by the
caller.

Reviewed By: ThomasRaoux, nicolasvasilache

Differential Revision: https://reviews.llvm.org/D133965