Handle alt on a _|_ - typed value

Return the result of the discriminant from trans_alt,
rather than nil, in the _|_ case. This was breaking the
enclosed test case (alt-bot-2) when optimization was disabled.

Closes #769

src/comp/middle/trans_alt.rs

@@ -433,7 +433,7 @@ fn trans_alt(cx: &@block_ctxt, expr: &@ast::expr, arms: &ast::arm[],
             ret rslt(cx, cx.build.Unreachable());
         }
         else {
-            ret rslt(cx, C_nil());
+            ret er;
         }
     }
 

src/test/run-pass/alt-bot-2.rs

@@ -0,0 +1,3 @@
+// n.b. This was only ever failing with optimization disabled.
+fn a() -> int { alt ret 1 { 2 { 3 } } }
+fn main() { a(); }