Added a few more global variable opcodes.

Gotta catch em all!

This time:

 PRIVATE_IF_DIRECT_GLOBAL and ASSIGN_PRIVATE_IF_DIRECT_GLOBAL

These will fetch or assign a global variable if the currently
executing program is the program the object is cloned from.

These are only slightly slower than the F_PRIVATE_GLOBAL family of
opcodes, and the overhead if the global is not actually private is
minimal.

Missing: [ASSIGN_]PRIVATE_IF_DIRECT_TYPED_GLOBAL[_AND_POP] and
ASSIGN_PRIVATE_IF_DIRECT_GLOBAL_AND_POP.