Fixes jruby#4596.

JRuby currently does not cache anything for method_missing calls
(i.e. calls to methods that do not exist on the target class's
method table). Instead, after discovering the missing method, it
proceeds down a slow lookup path, dispatching to method_missing
with a symbolic method name.

However this logic still attempts to acquire a SwitchPoint from
the target class, since that SP must be acquired before the
initial method lookup to ensure proper invalidation ordering.

Normally, this isn't a problem. If the class is still relatively
static, the SwitchPoint will be created once, and then the cost
of calling method_missing is the same as for non-indy. However for
type very rapidly. This led to a large number of unused
SwitchPoint being created, one for each class. And since the
method_missing path never cached anything, it never updated call
site thresholds, resulting in call sites that would continue this
logic forever.

Normal calls, if they fail repeatedly, will eventually fail the
entire site and revert to non-indy method dispatch (currently a
simple monomorphic cache).

This patch lifts the threshold-bumping out of the caching logic
so it can also be used by method_missing dispatches. This allows
repeated method_missing calls against different classes to also
fail the site, reverting it to non-indy behavior. With this patch,
the benchmark in jruby#4596 runs faster with indy than without, as
expected.