[PATCH] Remove copies of MCSubtargetInfo from MCRelaxableFragment

[Rafael Espíndola]

> So I guess it's probably worth untangling this further.  I could convert
> everything `encodeInstruction()` calls transitively to take a
> `FeatureBitset` instead of a `const MCSubtargetInfo &`.  Make sense?

BTW, it wouldn't surprise me if you could just pass the fragment
itself and store just a few booleans in it (IsMicroMips, IsThumb,
etc).

Cheers,
Rafael