Add assertions for matches that used to use analyze_branch (#5733)

Following up from #5730, add debug assertions to ensure that new branch instructions don't slip through matches that used to use analyze_branch.

cranelift/codegen/src/flowgraph.rs

@@ -141,7 +141,7 @@ impl ControlFlowGraph {
                         self.add_edge(block, inst, *dest);
                     }
                 }
-                _ => {}
+                inst => debug_assert!(!inst.opcode().is_branch()),
             }
         }
     }