A Downcast is now reached when const-checking a for loop

I believe this occurs because the old checker stopped processing basic
blocks after a `SwitchInt`.

Author: ecstatic-morse

src/test/compile-fail/consts/const-fn-error.rs

@@ -9,6 +9,7 @@ const fn f(x: usize) -> usize {
         //~| ERROR E0017
         //~| ERROR E0080
         //~| ERROR E0744
+        //~| ERROR E0019
         sum += i;
     }
     sum

src/test/compile-fail/issue-52443.rs

@@ -11,4 +11,5 @@ fn main() {
     //~| ERROR `for` is not allowed in a `const`
     //~| ERROR references in constants may only refer to immutable values
     //~| ERROR evaluation of constant value failed
+    //~| ERROR constant contains unimplemented expression type
 }