RFR: 8225179: (regex) Minor Pattern cleanup

[Claes Redestad]

Hi,

please review this j.u.regex.Pattern cleanup.

- refactor BitClass to be a BmpCharPredicate (which allows removing
two identical(!) lambdas), which improves startup and reduces
allocations when compiling Patterns.
- remove unused GroupRef class
- made anonymous lookbehindEnd Node instance into an explicit class
which will be lazily rather than eagerly loaded
- various cleanups of unused variables, methods and redundant
inititialization

Webrev: http://cr.openjdk.java.net/~redestad/8225179/open.00/
Bug:    https://bugs.openjdk.java.net/browse/JDK-8225179

Testing: tier1-3

Thanks!

/Claes