Shrink PL_op_private_bitdefs

It doesn’t matter whether things in this table are ordered by opcode,
because the indices into it are stored in PL_op_private_bitdef_ix.

If we have multiple ops with exactly the same private flags, we don’t
need multiple entries in PL_op_private_bitdefs.

One practical advantage is that patches are less likely to conflict,
which will make rebasing easier.  (I hope.)