i965/fs: Invalidate live intervals after copy propagation.

For copy propgation, we've dropped the use of a GRF in favor of a
(probably later) use of a different GRF.  This definitely requires
invalidating intervals.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/mesa/drivers/dri/i965/brw_fs_copy_propagation.cpp

@@ -160,5 +160,8 @@ fs_visitor::opt_copy_propagate()
 
    ralloc_free(mem_ctx);
 
+   if (progress)
+      live_intervals_valid = false;
+
    return progress;
 }