Make sure we double back after legalizing an instruction.

The other legalizer cases have a continue after setting the position
to double back, while this one didn't.  Make sure that we do, in case
another legalizer block is added after this one.

lib/cretonne/src/legalizer/mod.rs

@@ -91,6 +91,7 @@ pub fn legalize_function(func: &mut Function, cfg: &mut ControlFlowGraph, isa: &
                     // unsound. Should we attempt to detect that?
                     if changed {
                         pos.set_position(prev_pos);
+                        continue;
                     }
                 }
             }