Fix incorrect double assignment in MIR for while loops

src/test/ui/nll/assign-while-to-immutable.rs

@@ -0,0 +1,11 @@
+// We used to incorrectly assign to `x` twice when generating MIR for this
+// function, preventing this from compiling.
+
+// check-pass
+
+fn main() {
+    let x = while false {
+        break;
+    };
+    let y = 'l: while break 'l {};
+}