JIT: don't reuse box temps when optimizing (#13703)

JIT: don't reuse box temps when optimizing (#13703)

The importer reuses temps for different box operations that do not overlap.
This keeps the number of temps to a minimum and reduces prolog zeroing if the
temps end up untracked. But reuse prevents the importer from accurately typing
the temp.

So now, when optimizing, allocate a new temp for each box operation, and type
the temp with the type of the box.

This, along with a small update in `gtGetClassHandle` to obtain the box type,
enables some devirtualization of interface calls on boxes and will facilitate
future changes to optimize away boxes entirely (eg #5626).