r300/compiler: Disable register rename pass on r500

r300/compiler: Disable register rename pass on r500

The scheduler and the register allocator are not good enough yet to deal
with the effects of the register rename pass.  This was causing a 50%
performance drop in Lightsmark.  The pass can be re-enabled once the
scheduler and the register allocator are more mature.  r300 and r400
still need this pass, because it prevents a lot of shaders from using
too many texture indirections.

NOTE: This is a candidate for the 7.10 branch.