Refactoring of struct promotion code for the future fix. (#20216)

Refactoring of struct promotion code for the future fix. (#20216)

Create StructPromotionHelper.

* Add the check for promoted fields with changed type.

`CanPromoteStructType` can fake a field type if the field type is "structs of a single field of scalar types aligned at their natural boundary.". Add a check in debug that when we morph struct field access we have an expected type in the field handle.