Introduce loop coalescing utility and a simple pass

Multiple (perfectly) nested loops with independent bounds can be combined into
a single loop and than subdivided into blocks of arbitrary size for load
balancing or more efficient parallelism exploitation.  However, MLIR wants to
preserve the multi-dimensional multi-loop structure at higher levels of
abstraction. Introduce a transformation that coalesces nested loops with
independent bounds so that they can be further subdivided by tiling.

PiperOrigin-RevId: 258151016