Add regression test

tests/cases/compiler/controlFlowWithIncompleteTypes.ts

@@ -0,0 +1,27 @@
+// Repro from #11000
+
+declare var cond: boolean;
+
+function foo1() {
+    let x: string | number | boolean = 0;
+    while (cond) {
+        if (typeof x === "string") {
+            x = x.slice();
+        }
+        else {
+            x = "abc";
+        }
+    }
+}
+
+function foo2() {
+    let x: string | number | boolean = 0;
+    while (cond) {
+        if (typeof x === "number") {
+            x = "abc";
+        }
+        else {
+            x = x.slice();
+        }
+    }
+}