[interp] Outline box_nullable. (#16356)
Without being entirely scientific about it, this case seems
to be one that uses more than typical locals and therefore
be on a sort of critical path to reduce frame size.
I have a change that reduces frame to 0x78 bytes (albeit
not yet working) and to achieve that, in this case I refetched
the locals after function calls.
This achieves similar but perhaps more elegantly.
If we rest at larger frame size then this case might not be on the critical path.
https://github.com/mono/mono/issues/16172.