FFR ops (SETFFR/RDFFR/WRFFR) and BRKN (destructive, Pdm re-packs Pd) via
specgen; CPY (predicated from GPR), EXT (destructive, imm8 split via new
SVE_EXT_IMM), MOV-predicated (=SEL with Zm=Zd, via ZD_ZM_DUP), and the
predicate aliases NOT/MOVS/MOV (EOR/ORR/AND with a duplicated predicate
field, via PG4_PM_DUP/PN_PM_DUP/PN_PG_PM_DUP). All byte-exact vs llvm-mc;
the predicate aliases decode to their canonical base op (identical bytes,
as expected). 461 tests green.
(SVE_XAR_Z deferred: its tsz:imm3 shift field does not follow the NEON
immh:immb scheme and needs a bespoke esize-from-Z encoder.)
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