RFR: 8242115: C2 SATB barriers are not safepoint-safe

[Igor Veresov]

Implement load pinning, use it for g1 pre-value loads, add verification that the load control dependency is kept and that there are no safepoints between the load of the pre-value and the marking check (with the exception of the CAS intrinsics where it's permitted).

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Commit messages:
 - Implement load pinning, use it for g1 pre-val loads, add verification.

Changes: https://git.openjdk.org/jdk/pull/10517/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10517&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8242115
  Stats: 185 lines in 6 files changed: 172 ins; 2 del; 11 mod
  Patch: https://git.openjdk.org/jdk/pull/10517.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10517/head:pull/10517

PR: https://git.openjdk.org/jdk/pull/10517