instructions-aarch64: Handle destructive EXT in CanTakeSVEMovprfx (#55)

The ARM architecture manual states that a destructive SVE EXT
instructions might be preceded by a MOVPRFX instruction, and that this
is allowed, but that:

1. The MOVPRFX instruction must be unpredicated
2. The MOVPRFX instruction must specify the same destination as the EXT
   instruction.
3. The destination register cannot refer to any other source operand.