RFR: 8250745: Fix a potential bug on AVX512 machines with assert(eval_map.contains(n)) failed: absent

[jiefu(傅杰)]

Hi all,

This bug[1] was first observed while testing Panama's vector api on AVX512 machines.
During the discussion on panama-dev, Vladimir Ivanov pointed out that this is not Vector API-specific[2].
So it would be better to fix the potential bug in the auto-vectorizer.

JBS:    https://bugs.openjdk.java.net/browse/JDK-8250745
Webrev: http://cr.openjdk.java.net/~jiefu/8250745/webrev.00/

Testing:
  1. jdk/jdk: tier1-3 on Linux/x64 AVX512 machines
  2. Panama(vectorIntrinsics): jdk/incubator/vector on Linux/x64 AVX512 machines

Thanks a lot.
Best regards,
Jie

[1] https://bugs.openjdk.java.net/browse/JDK-8250675
[2] https://mail.openjdk.java.net/pipermail/panama-dev/2020-July/010113.html