r300/compiler: Implement the texture semaphore

r300/compiler: Implement the texture semaphore

The texture semaphore allows for prefetching of texture data.  On my
RV515, this increases the FPS of Lightsmark by 33% (This is with the
reg_rename pass enabled, which is enabled in the next commit).

There is a new env variable now called RADEON_TEX_GROUP, which allows
you to specify the maximum number of texture lookups to do at once.
The default is 8, but different values could produce better results
for various application / card combinations.