[mlir][scf] Implement getSingle... of LoopLikeOpInterface for scf::ForallOp (#67883)

The `getSingle(IterationVar|UpperBound|LowerBound|Step)` methods of
`LoopLikeOpInterface` are useful to quickly query the iteration space of 
unidimensional loops. Until now, `scf::ForallOp` always fell back to the
default implementation of these methods, returning `std::nullopt`.

This patch implements those methods, returning the respective bounds
or steps in the special case of `rank == 1`.