This patch adds addsub_imm8_opt_lsl_(i8|i16|i32|i64) operands
that are unsigned values in the range 0 to 255. For element widths of
16 bits or higher it may also be a signed multiple of 256 in the
range 0 to 65280.
Note: This also does some refactoring to reuse convenience function
getShiftedVal<shift>(), and now allows AArch64 scalar 'ADD #-4096' to be
accepted to be mapped to SUB #4096.