target/arm: Introduce PREDDESC field definitions

target/arm: Introduce PREDDESC field definitions

SVE predicate operations cannot use the "usual" simd_desc
encoding, because the lengths are not a multiple of 8.
But we were abusing the SIMD_* fields to store values anyway.
This abuse broke when SIMD_OPRSZ_BITS was modified in e2e7168a214.

Introduce a new set of field definitions for exclusive use
of predicates, so that it is obvious what kind of predicate
we are manipulating.  To be used in future patches.

Cc: [email protected]
Signed-off-by: Richard Henderson <[email protected]>
Message-id: 20210113062650[email protected]
Reviewed-by: Peter Maydell <[email protected]>
Signed-off-by: Peter Maydell <[email protected]>