Fix defined?(::Object) logic in JIT. Partial fix for #2090.

The current IR for this logic uses the runtime helper call
isDefinedConstantOrMethod, which can trigger exceptions on its own
(if e.g. the method lookup triggers an error?). Because of that
handler, this bug in the JIT was caught and swallowed, resulting
in the defined? returning nil. That is not correct behavior; only
the exceptions we would expect to be raised by these lookups
should be caught.

In addition, I don't believe the :: form can be used to call
methods (::foo doesn't parse), so the use of exception handling
and isDefinedConstOrMethod may be inappropriate here.