[mlir][SCF] Peel scf.for loops for even step divison

Add ForLoopBoundSpecialization pass, which specializes scf.for loops into a "main loop" where `step` divides the iteration space evenly and into an scf.if that handles the last iteration.

This transformation is useful for vectorization and loop tiling. E.g., when vectorizing loads/stores, programs will spend most of their time in the main loop, in which only unmasked loads/stores are used. Only the in the last iteration (scf.if), slower masked loads/stores are used.

Subsequent commits will apply this transformation in the SparseDialect and in Linalg's loop tiling.

Differential Revision: https://reviews.llvm.org/D105804