RFR: 8318509: x86 count_positives intrinsic broken for -XX:AVX3Threshold=0

[Claes Redestad]

The AVX-512 implementation of count_positives is broken when there are negative values in the tail block, since the len register is repurposed to then re-read as-if it still contained the original value. 

Proposed fix is to restore len from the value in tmp1, which at this point holds the count of bytes in the tail. 

Existing TestCountPositives test is pretty exhaustive and just needs to be run explicitly with AVX3Threshold=0 enabled.

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Commit messages:
 - Add test run with AVX3Threshold=0 to TestCountPositives test
 - Merge branch 'master' into avx3_countPositives
 - Fix AVX3 implementation of countPositives stub

Changes: https://git.openjdk.org/jdk/pull/16267/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16267&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8318509
  Stats: 32 lines in 2 files changed: 22 ins; 2 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/16267.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16267/head:pull/16267

PR: https://git.openjdk.org/jdk/pull/16267