JIT: cleanups/optimizations for ps loadstore

I'm not quite sure why the float paired stores were written how they were,
but it should be more consistent now.

Also get rid of the use of a psTemp global that wasn't really needed.

Add some comments.

@JMC47 this needs some testing in a few games to make sure I didn't horribly break anything with the float paired store change
Yes, you're right... those are potential uses.  Part of it is probably just luck that nobody wrote code like that.