JIT: add extra check to struct of struct of x promotion (dotnet/coreclr#19156)

JIT: add extra check to struct of struct of x promotion (dotnet/coreclr#19156)

Avoid promoting structs that contain struct fields that themselves
wrap single simple fields, if those single simple fields are smaller
than their enclosing struct.

Otherwise we run the risk of losing track of the "extra" bytes in the
innner struct.

Addresses dotnet/coreclr#19149.

Commit migrated from https://github.com/dotnet/coreclr/commit/d38774d80e07436eadc28f7255a8c9f8edb1e570