Add branch order optimization to simple_preopt to encourage fallthroughs.

cranelift/codegen/src/context.rs

@@ -225,7 +225,7 @@ impl Context {
 
     /// Perform pre-legalization rewrites on the function.
     pub fn preopt(&mut self, isa: &TargetIsa) -> CodegenResult<()> {
-        do_preopt(&mut self.func);
+        do_preopt(&mut self.func, &mut self.cfg);
         self.verify_if(isa)?;
         Ok(())
     }