i965: Add envvar to debug the optimization passes.

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

src/mesa/drivers/dri/i965/intel_debug.c

@@ -64,6 +64,7 @@ static const struct dri_debug_control debug_control[] = {
    { "no16",  DEBUG_NO16 },
    { "blorp", DEBUG_BLORP },
    { "nodualobj", DEBUG_NO_DUAL_OBJECT_GS },
+   { "optimizer", DEBUG_OPTIMIZER },
    { NULL,    0 }
 };