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.
Do we want to update the ranges in the diagnostics? [-255, 255] here for instance.