This patch adds the appropriate macros, tests and codec entries to
encode the following variants:
```
BFCVT   <Hd>, <Sn>
BFCVTN  <Vd>.4H, <Vn>.4S
BFCVTN2 <Vd>.8H, <Vn>.4S
BFDOT   <Vd>.<Ts>, <Vn>.<Tb>, <Vm>.<Tb>
BFDOT   <Vd>.<Ts>, <Vn>.<Tb>, <Vm>.2H[<index>]
BFMLALB <Vd>.4S, <Vn>.8H, <Vm>.8H
BFMLALB <Vd>.4S, <Vn>.8H, <Vm>.H[<index>]
BFMLALT <Vd>.4S, <Vn>.8H, <Vm>.8H
BFMLALT <Vd>.4S, <Vn>.8H, <Vm>.H[<index>]
BFMMLA  <Vd>.4S, <Vn>.8H, <Vm>.8H
```

Issue #2626