Clean up some NaN handling in some instructions

I had looked at some of the VCMP usage in armjit and did some tests to verify what should be correct.

vmin/vmax look tricky for proper NaN handling.  Added notes in place of the old ones, but basically: -NAN < -INF < real < INF < NAN.  I did at least fix 0 handling (take t's sign.)

Otherwise, this fixes the sat0/sat1 operations.  On x86 this adds a few more instructions, but I'm worried if NaN handling mattered for Peace Walker, it may matter elsewhere.

-[Unknown]