ppc: extend opindex to 16 bits

ppc: extend opindex to 16 bits

With the upcoming SVP64 extension[0] to PowerPC architecture, it became
evident that PowerPC operand indices no longer fit 8 bits. This patch
switches the underlying type to uint16_t, also introducing a special
typedef so that any future extension goes even smoother.

[0] https://libre-soc.org