RFR: 8238684: Override getOrDefault in immutable Map implementations

[Claes Redestad]

Hi,

in our immutable map implementations, it's unnecessary for getOrDefault
to check containsKey(key) in case get(key) returns null. This follows
since null values are disallowed.

Overriding the method with an implementation is a reasonable
optimization, for example a 50% improvement on the added
microbenchmark. I found this opportunity when looking at startup
profiles, so yes, there's a tiny startup improvement, too. :-)

Webrev: http://cr.openjdk.java.net/~redestad/8238684/open.00/
Bug:    https://bugs.openjdk.java.net/browse/JDK-8238684

Testing: tier1+2 (ongoing)

Thanks!

/Claes