[sve] Disallow dup with shift on byte-sized lanes

The Arm ARM states dup-immediate with shift on byte-sized lanes is an undefined
encoding, so add assertions in the assembler to disallow this, and handle
the encoding appropriately in the disassembler and simulator.

Change-Id: Ibce08d2577e2b780dbb7e4fa478d2f8d18e99a39