RFR: 8236641: Improve Set.of(...).iterator() warmup characteristics

[Claes Redestad]

Hi,

we're doing plenty of iterations over Set.of() instances during
bootstrap, which makes these operations show up prominently in
startup profiles. This patch refactors a few hot methods to get a
measureable startup improvement without regressing on targeted
microbenchmarks.

Bug:    https://bugs.openjdk.java.net/browse/JDK-8236641
Webrev: http://cr.openjdk.java.net/~redestad/8236641/open.00/

(The patch is baselined against 8236850)

Thanks!

/Claes