Fix cfg bug if for loops never exit normally

ex:

for i in xrange(5):
    pass
else:
    return 2
"unreachable here!"

We didn't prune the "unreachable here" line, which caused
some issues.  I guess we could also support compiling
trivially-unreachable blocks.

src/core/cfg.cpp

@@ -1909,8 +1909,13 @@ public:
             curblock->connectTo(end_block);
         }
 
-        cfg->placeBlock(end_block);
-        curblock = end_block;
+        if (end_block->predecessors.size() == 0) {
+            delete end_block;
+            curblock = NULL;
+        } else {
+            cfg->placeBlock(end_block);
+            curblock = end_block;
+        }
 
         return true;
     }
@@ -2436,8 +2441,13 @@ CFG* computeCFG(SourceInfo* source, std::vector<AST_stmt*> body) {
                    || terminator->type == AST_TYPE::Raise);
         }
 
-        if (b->predecessors.size() == 0)
-            assert(b == rtn->getStartingBlock());
+        if (b->predecessors.size() == 0) {
+            if (b != rtn->getStartingBlock()) {
+                rtn->print();
+                printf("%s\n", source->getName().c_str());
+            }
+            ASSERT(b == rtn->getStartingBlock(), "%d", b->idx);
+        }
     }
 
     // We need to generate the CFG in a way that doesn't have any critical edges,

test/tests/for.py

@@ -34,3 +34,12 @@ else:
 for i in xrange(5):
     print i
     break
+
+def f():
+    for i in xrange(5):
+        if i == 10:
+            return 2
+    else:
+        return 5
+
+f()