[AArch64][SVE] Asm: Fix supported immediates for DUP/CPY

This patch fixes an issue in the implementation of DUP/CPY where certain
immediates were not accepted. Immediates should be interpreted as a two's
complement encoding of a value that fits the number of bits of the element
type.

          mov z0.b, p0/z, #127
     <=>  mov z0.b, p0/z, #-129
     <=>  mov z0.b, p0/z, #0xffffffffffffff7f

This behaviour is in line with the GNU assembler.

Reviewed By: c-rhodes

Differential Revision: https://reviews.llvm.org/D94776