ppc/svp64: introduce non-zero operand flag

svstep and svshape instructions subtract 1 before encoding some of the
operands. Obviously zero is not supported for these operands. Whilst
PPC_OPERAND_PLUS1 fits perfectly to mark that maximal value should be
incremented, there is no flag which marks the fact that zero values are
not allowed. This patch adds a new flag, PPC_OPERAND_NONZERO, for this
purpose.