Fix unreachability of elif blocks (python#1455)

Fix python#1454.

Author: JukkaL

mypy/semanal.py

@@ -2644,7 +2644,7 @@ def infer_reachability_of_if_statement(s: IfStmt,
             # This condition is always true, so all of the remaining
             # elif/else bodies will never be executed.
             for body in s.body[i + 1:]:
-                mark_block_unreachable(s.body[i])
+                mark_block_unreachable(body)
             if s.else_body:
                 mark_block_unreachable(s.else_body)
             break

mypy/test/data/check-unreachable-code.test

@@ -98,3 +98,16 @@ if PY3:
 else:
     class X(object):
         pass
+
+[case testUnreachabilityAndElifPY3]
+def f(): pass
+PY3 = f()
+if PY3:
+    pass
+elif 1:
+    import nonexistent
+    1 + ''
+else:
+    import bad_name
+    1 + ''
+[out]