[jdk15u-dev] RFR: 8267652: c2 loop unrolling by 8 results in reading memory past array

[Vladimir Kempik]

Please review this backport to jdk15u-dev
it wasn't applying clean due to the miss of 8223347 in 15u
8223347 is too big to be taken into older JDK, hence I took very small part of 8223347 and included into this backport
This small part moves the location of helper funtions ( like vector_length_in_bytes) in x86.ad
and slighlt changes their signature. it's necessary for predicate(vector_length_in_bytes()) construction to work.
testing - hotspot_tier1.

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Commit messages:
 - Backport dc12cb78b81f56e9d4b282cf7cad5faa9a9886bf

Changes: https://git.openjdk.java.net/jdk15u-dev/pull/78/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk15u-dev&pr=78&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8267652
  Stats: 122 lines in 1 file changed: 64 ins; 37 del; 21 mod
  Patch: https://git.openjdk.java.net/jdk15u-dev/pull/78.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk15u-dev pull/78/head:pull/78

PR: https://git.openjdk.java.net/jdk15u-dev/pull/78