JIT should use boxing helper to box structs with many GC pointers

Unoptimized code is faster than optimized code: https://github.com/dotnet/runtime/pull/101605#issuecomment-2080776277

https://github.com/dotnet/runtime/pull/101605#issuecomment-2079487090

Prototype with some extra discussion at https://github.com/dotnet/runtime/pull/101669.