Finish implementation of LOOPXY

[DerelictDrone]

LOOPXY was an unfinished instruction for 2D looping using the ECX and EDX registers, introduced during the vectors expansion, it took two pointers, we may never know how blackphoenix intended its implementation so this is my guess on how to make it useful from what little remnants there were of it in the code & documentation.

This is, so far, the only LOOP instruction that exits on -1 rather than 0, so it can access 0 indexes for things.

Pseudocode for how the instruction actually works:

Decrement EDX
if EDX == -1 Decrement ECX
if ECX == -1 exit loop, else JMP to operand 1 end
else JMP to operand 2 end

Usage example, iterating over 6 vector 4's to get an estimate of F=MA using the first three indices to get a constant velocity for A and the 4th as M

This may be a gimmicky instruction/usecase, but it's been compileable and useless for more than 5 years now, if desired by maintainers the instruction can be removed instead.