Fix #2261 Mark threadpoll with 'can_raise_exceptions'

* Tom will look into possibly removing some thread polls
  on scope entry.

* In any case, this won't block opts as much I was thought
  it might.

core/src/main/java/org/jruby/ir/Operation.java

@@ -163,7 +163,7 @@ public enum Operation {
     RESCUE_EQQ(OpFlags.f_can_raise_exception), // a === call used in rescue
     RUNTIME_HELPER(OpFlags.f_has_side_effect | OpFlags.f_can_raise_exception),
     SET_CAPTURED_VAR(OpFlags.f_can_raise_exception),
-    THREAD_POLL(OpFlags.f_is_book_keeping_op | OpFlags.f_has_side_effect),
+    THREAD_POLL(OpFlags.f_is_book_keeping_op | OpFlags.f_has_side_effect | OpFlags.f_can_raise_exception),
     THROW(OpFlags.f_has_side_effect | OpFlags.f_can_raise_exception | OpFlags.f_is_exception),
     // FIXME: TO_ARY is marked side-effecting since it can allocate new objects
     // Clarify semantics of 'f_has_side_effect' better