Use the correct prestate for calls

The prestate for calls was getting set incorrectly to the poststate for the operator
in the call. This worked before since most of the time, operator expressions are
pure. Issue 1895 shows how this breaks when the operator is a closure that has a
move-in capture clause.

(I had a several-day, multi-file patch for this that didn't work... and then it
turned out to be a one-line fix. The joys of programming.)

Closes #1895

src/comp/middle/tstate/states.rs

@@ -190,7 +190,7 @@ fn find_pre_post_state_call(fcx: fn_ctxt, pres: prestate, a: @expr,
                                         %u exprs vs. %u ops",
                                        vec::len(bs), vec::len(ops)]);
     }
-    ret find_pre_post_state_exprs(fcx, expr_poststate(fcx.ccx, a), id, ops,
+    ret find_pre_post_state_exprs(fcx, pres, id, ops,
                                    bs, cf) || changed;
 }
 

src/test/run-pass/issue-1895.rs

@@ -0,0 +1,7 @@
+fn main() {
+  let x = 1;
+  let y = fn@[move x]() -> int {
+             x
+          }();
+}
+