AArch64: sxtl{2}, uxtl{2} aliases not used

[adamjseitz]

The aliases sxtl{2} and uxtl{2} are preferred over SSHLL{2} <Vd>.<Ta>, <Vn>.<Tb>, #0 and USHLL{2} <Vd>.<Ta>, <Vn>.<Tb>, #0.

New test cases:

!# sxtl/uxtl aliases
!# CS_ARCH_ARM64, CS_MODE_ARM, CS_OPT_DETAIL
0x20,0xa4,0x08,0x0f == sxtl v0.8h, v1.8b ; operands[0].vas: 0x5 ; operands[1].vas: 0x2

!# sxtl/uxtl aliases
!# CS_ARCH_ARM64, CS_MODE_ARM, CS_OPT_DETAIL
0x20,0xa4,0x08,0x4f == sxtl2 v0.8h, v1.16b ; operands[0].vas: 0x5 ; operands[1].vas: 0x1

!# sxtl/uxtl aliases
!# CS_ARCH_ARM64, CS_MODE_ARM, CS_OPT_DETAIL
0x20,0xa4,0x08,0x2f == uxtl v0.8h, v1.8b ; operands[0].vas: 0x5 ; operands[1].vas: 0x2

!# sxtl/uxtl aliases
!# CS_ARCH_ARM64, CS_MODE_ARM, CS_OPT_DETAIL
0x20,0xa4,0x08,0x6f == uxtl2 v0.8h, v1.16b ; operands[0].vas: 0x5 ; operands[1].vas: 0x1