Cache Native/Overide alt lookups.

Looking at a startup profile, the following checks stand out within VM.execute:

- Override.hasMethod (450ms)
- ACCESS_FLAGS.isNative (400ms)

Caching the method lookup avoids these checks.
I think setting a previously-undeclared property on a constructed object is expensive, so we should pre-declare *alt* on methodInfo object construction, even if we don't set it until this code.