Created by: joshua-warburton
This patch adds the appropriate macros, tests and codec entries to encode the following variants:
SXTB <Zd>.<Ts>, <Pg>/M, <Zn>.<Ts>
SXTH <Zd>.<Ts>, <Pg>/M, <Zn>.<Ts>
SXTW <Zd>.D, <Pg>/M, <Zn>.D
UXTB <Zd>.<Ts>, <Pg>/M, <Zn>.<Ts>
UXTH <Zd>.<Ts>, <Pg>/M, <Zn>.<Ts>
UXTW <Zd>.D, <Pg>/M, <Zn>.D
issues: #3044