[WINED3D 7/7] Fix lookup for LOOP register

[Ivan Gyurdiev]

Jason Green wrote:
> - The LOOP register is an extended register and wasn't being checked
> for in the current code.  This moves the check up a level and checks
> against the instruction code which is more efficient anyway.
It's important to also correct the reason why it wasn't being detected 
where it was - instead of using the REGTYPE mask, we should call the 
shader_get_regtype function, which takes into account extended registers.