Tests for while loops that may invalidate constraints

Wrote some small test cases that use while loops and moves, to
make sure the poststate for the loop body gets propagated into the
new prestate and deinitialization gets reflected.

Along with that, rewrite the code for intersecting states. I still
find it dodgy, but I guess I'll continue trying to add more tests.
Also, I'll probably feel better about it once I start formalizing
the algorithm.

src/test/compile-fail/do-while-constraints.rs

@@ -0,0 +1,17 @@
+// xfail-stage0
+// error-pattern: Unsatisfied precondition constraint (for example, init(y
+fn main() {
+
+  let int y = 42;
+  let int x;
+  do {
+    log y;
+    do {
+      do {
+	do {
+	  x <- y;
+	} while (true);
+      } while (true);
+    } while (true);
+  } while (true);
+}