[mlir][ArmSME] Refactor ArmSMEToSCF to used shared loop-building helper (NFC)

This will make fixing a bug (next patch) a change to one place, rather
than fixing three separate rewrites.

Note: `TileLoadOpWithMaskAndPadZeroConversion` has been merged into
`TileLoadOpConversion`, since after this change those two rewrites were
pretty much identical.

Author: MacDue