[PowerPC] Remove symbolLo/symbolHi instruction operand types

Now that there is no longer any distinction between symbolLo
and symbolHi operands in either printing, encoding, or parsing,
the operand types can be removed in favor of simply using
s16imm.

This completes the patch series to decouple lo/hi operand part
processing from the particular instruction whose operand it is.

No change in code generation expected from this patch.