This fixes a crash in lowering, at least (though only initializes the
vptr to
null for now) - certainly open to naming feedback on the instruction, or
the
exact semantics (we could have a global vptr instruction that's
referenced from
the existing instructions for reading globals, for instance).
I guess we'll want one type parameter for the vptr_init instruction,
which is
the type that this is a vptr for? (can do that here or in a follow-on
patch)