RFR: 8200289: Reduce number of exceptions created when calling Lookup::canBeCached

[Claes Redestad]

Hi,

a simple refactoring of Lookup::canBeCached avoids throwing ~315 IAEs on 
nashorn/jjs bootstrap, and likely helpful elsewhere too.

Bug: https://bugs.openjdk.java.net/browse/JDK-8200289

Webrev: http://cr.openjdk.java.net/~redestad/8200289/open.00/

Thanks!

/Claes