ZJIT: Implement polymorphic GetIvar support

[tekknolagi]

Create some Compare+IfTrue sequences for the type/shape, branch to a block, and branch back

[tekknolagi]

We can start off with a really crappy C stub that takes a slab of memory with shape/offset pairs and loops over it. This would avoid the branch issue

[tekknolagi]

We could also add a field to GetIvar that is like "speculated shape" which might be less finicky than RefineShape

[XrXr]

We decided to implement this during HIR building, which doesn't need to make cross BB reference. So no longer blocked by new reg alloc.

[XrXr]

Unassigning myself since this proved to not give a noticeable speed boost at this point.