RFR(XS): 8241365: Define Unique_Node_List::contains() to prevent usage by mistake

[Roland Westrelin]

Thanks for the review.

> Good. What testing sis you run?

Some CTW testing, hotspot/jtreg/compiler hotspot/jtreg/gc
hotspot/jtreg/runtime jdk/java/lang jdk/java/util with -Xcomp.

Roland.