RFR: 8245677: Optimize lookups in empty HashMap

[Claes Redestad]

Hi,

sponsoring this patch from Christoph Dreis.

By pushing down the hash calculation we can optimize lookups
in empty HashMaps (also helps LinkedHashMap).

Bug:    https://bugs.openjdk.java.net/browse/JDK-8245677
Webrev: http://cr.openjdk.java.net/~redestad/8245677/open.00/

Testing: tier1+2

Original report: 
https://mail.openjdk.java.net/pipermail/core-libs-dev/2020-May/066530.html

/Claes