MVT-based vectors: first stab

[Roland Westrelin]

Hi Rémi,

> why a __Value is not represented by two fields too ?
>
> A __Value is a value type with no statically known type so if can not be represented by two fields, it should be represented by 3 fields (2 fields + a pointer to the type).

Why 2 fields?

For:

class SomeValue {
  double x;
  double y;
  double z;
}

the JIT uses (x, y, z) as an internal representation of the value.

Roland.