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
Pretty sure it's just because no game does this; if a game isn't using virtual memory, the only way for a memory access to be outside of RAM is memory-mapped I/O, and those values aren't floating-point.  It would be nice to fix this, though, if only to make it harder to crash the emulator.

In terms of the performance hit... probably visible unless we fix quantized loads/stores to support fastmem.