aarch64: Add the SME2 predicate-related instructions

Implementation-wise, the main things to note here are:

- the WHILE* instructions have forms that return a pair of predicate
  registers.  This is the first time that we've had lists of predicate
  registers, and they wrap around after register 15 rather than after
  register 31.

- the predicate-as-counter WHILE* instructions have a fourth operand
  that specifies the vector length.  We can treat this as an enumeration,
  except that immediate values aren't allowed.

- PEXT takes an unsuffixed predicate index of the form PN<n>[<imm>].
  This is the first instance of a vector/predicate index having
  no suffix.