RFR: 8358568: C2 compilation hits "must have a monitor" assert with -XX:-GenerateSynchronizationCode

[hanguanqiang]

This PR fixes JDK-8358568, a JVM crash triggered when running with -XX:-GenerateSynchronizationCode

Problem：
When synchronization code generation is disabled by -XX:-GenerateSynchronizationCode, the compiler’s do_monitor_exit() method still tries to access monitor objects without checking if any monitors exist.This causes an assertion failure and JVM crash.

Root Cause：
Parse::do_monitor_exit() calls shared_unlock() using monitor info unconditionally,but with GenerateSynchronizationCode disabled, no monitor info is available, leading to invalid access.

Fix
Add a check in do_monitor_exit() to skip monitor unlocking if GenerateSynchronizationCode is false, avoiding invalid monitor access and preventing the crash.

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Commit messages:
 - 8358568: C2 compilation hits "must have a monitor" assert with -XX:-GenerateSynchronizationCode

Changes: https://git.openjdk.org/jdk/pull/26108/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26108&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8358568
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/26108.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26108/head:pull/26108

PR: https://git.openjdk.org/jdk/pull/26108