RFR: 8214761: Bug in parallel Kahan summation implementation

[Ian Graves]

Fixes a bug where the compensated sum should be negated when added together in the merge step of a given collector. This impacts accuracy of parallel summations with Double streams and creates larger deviations from a standard sequential (ie non-parallel) compensated summation.

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Commit messages:
 - Fixing a compensated/Kahan summation bug that increase error

Changes: https://git.openjdk.java.net/jdk/pull/3442/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3442&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8214761
  Stats: 16 lines in 3 files changed: 10 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3442.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3442/head:pull/3442

PR: https://git.openjdk.java.net/jdk/pull/3442