RFR: 8306732: TruncatedSeq::predict_next() attempts linear regression with only one data point

[Axel Boldt-Christmas]

TruncatedSeq::predict_next() attempts linear regression with only one data point, this leads to a division by zero. (There are infinit many linear functions that fit equally well for a single point).

I suggest we do what we do for the zero points case, namely pick one of the linear functions.

For zero points the current version picks `y = 0 + 0*x` and the suggestion is that for one point `P` the function `y = P_y + 0*x` is picked.

Tested for ZGC tier1-7 on Oracle supported platforms. Only ZGC use TruncatedSeq::predict_next()

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Commit messages:
 - 8306732: TruncatedSeq::predict_next() attempts linear regression with only one data point

Changes: https://git.openjdk.org/jdk/pull/13614/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13614&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8306732
  Stats: 8 lines in 1 file changed: 7 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/13614.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13614/head:pull/13614

PR: https://git.openjdk.org/jdk/pull/13614