Stop inferring bot/static when types/regions are unconstrained.

Also, some other changes that came up along the way:
- add a 'blk' region for the current block.
- detect unused type/region variables.

src/test/run-pass/unreachable-code.rs

@@ -1,57 +0,0 @@
-// xfail-pretty
-
-fn id(x: bool) -> bool { x }
-
-fn call_id() {
-    let c <- fail;
-    id(c);
-}
-
-fn call_id_2() { id(true) && id(ret); }
-
-fn call_id_4() { while id(ret) { } }
-
-fn bind_id_1() { bind id(fail); }
-
-fn bind_id_2() { bind id(ret); }
-
-fn fail_fail() { fail fail; }
-
-fn log_fail() { log(error, fail); }
-
-fn log_ret() { log(error, ret); }
-
-fn log_break() { loop { log(error, break); } }
-
-fn log_cont() { do { log(error, cont); } while false }
-
-fn ret_ret() -> int { ret 3 + (ret 2); }
-
-fn ret_guard() {
-    alt check 2 {
-      x if (ret) { x; }
-    }
-}
-
-fn rec_ret() { let _r: {c: int} = {c: ret}; }
-
-fn vec_ret() { let _v: [int] = [1, 2, ret, 4]; }
-
-fn fail_then_concat() {
-    let mut x = [], y = [3];
-    fail;
-    x += y;
-    "good" + "bye";
-}
-
-fn main() {
-  // Call the functions that don't fail.
-  rec_ret();
-  vec_ret();
-  ret_ret();
-  log_ret();
-  call_id_2();
-  call_id_4();
-  bind_id_2();
-  ret_guard();
-}