Fix the OpenGL depth buffer values from the vertex shader.
I am not sure i am understanding what the pipeline really does, and more so what
the GC/WII expects here. If my comments are incorrect, please let me know.
This was tested with MP2:E, ZWW on r6075.
Fixed some mistakes from r6076:
* Fixed missing UnlockAllX
* Fixed 64-bits compilation
* Improved constants folding in load instructions
* Merged load instructions
* Fixed the register allocator so that it can restore the registers state after jumping to the dispatcher (in case of a conditional jump)