Fix mir-cfg dumps

Fixes #81918
Fixes #82326 (duplicate)
Fixes #82325

src/test/ui/issues/issue-81918.rs

@@ -0,0 +1,11 @@
+// check-pass
+// dont-check-compiler-stdout
+// compile-flags: -Z unpretty=mir-cfg
+
+// This checks that unpretty=mir-cfg does not panic. See #81918.
+
+const TAG: &'static str = "ABCD";
+
+fn main() {
+    if TAG == "" {}
+}