mips: free up bits in insn_isa_mask

[Eric Christopher]

> > i'm adding support for another mips variant and we're running out of
> > bits.  this will free up unused ones.
> 
> There are eleven bits free in the upper half. Do you really need
> more than these? If yes, it might be the better for future
> expansion to separate the cpu specifics from the rest.
> 

We've gotten a _lot_ of slightly different variants over the years.  I'm
in the middle of cleaning up and contributing them.  And yes, we'd
already filled the last slot.  :)

Separating the cpu specifics from the rest would give us a few more bits
as well.

-eric


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