Add comment explaining why the model doesn't use vectors for registers

[Timmmm]

As far as I can see there's no reason to have this:

register x1  : regtype
register x2  : regtype
register x3  : regtype
register x4  : regtype
register x5  : regtype
register x6  : regtype
...

Instead of

register xrf : vec(regtype, 32)

This is done for all the X, F and V registers.

[Alasdair]

In the concurrency model, x1 .. xN are separate registers for intra-instruction dependencies (so instructions modifying different registers can be re-ordered), whereas xrf : vector(...) would be a single register.

[Timmmm]

Ah very subtle but that makes sense. I'll leave this issue open so we add a comment to the code.