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
I thought that paired singles can load u8/s8/u16/s16 values too, which is the idea being the dequantization?

Also, what about EFB accesses, that's not in RAM too, right?