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/ir/function.rs

@@ -315,7 +315,7 @@ impl FunctionStencil {
                 }
             }
 
-            _ => {}
+            inst => debug_assert!(!inst.opcode().is_branch()),
         }
     }