There are several issues with VMReg that mean it is unsuitable for our purposes.

- It can not distinguish between different register segments, meaning that for instance it can not distinguish between different vector register sizes.
- It only supports 32-bit stack slots, while some ABIs require more fine-grained stack slot addressing.

My current idea is to have a class like this:

    class VMStorage {
      char _type;
      char _reserved = 0;
      short _segment_mask;
      int _index;
    }

the `_type` and `_index` fields are just like the current `type` and `index` components in VMStorage. But, the `_segment_mask` field can be used to distinguish between different overlapping registers. For instance, RAX would be `_type = 0, _segment_mask = 0b0000_0000_0000_1111, _index = 0`. while EAX would be `_type = 0, _segment_mask = 0b0000_0000_0000_0111, _index = 0`, i.e. same type and index, just a different mask.

Supporting this would require re-writing the shuffling code.