[interp] Improve boxing/unboxing (#14048)

[interp] Improve boxing/unboxing (#14048)

* [interp] Remove redundant check

It can't be an enum if it is stored on the vtstack

* [interp] Optimize boxing

Makes boxing of primitives 4x faster and boxing of vt 3x faster.

* [interp] Remove unneccessary check in unbox

We do the same check as jit. Makes unboxing 10x faster.

* [interp] Extract some duplicate code for boxing

* [interp] Fix boxing of nullable with constrained calls

Before the box opcode optimization, it was handling nullable. When normally boxing a nullable we call the Box method, but this is awkward when doing a constrained call, so we just add a separate opcode. This is slightly slower than calling Box, but much faster than the old code using the generic mono_value_box_checked.