RFR: 8316746: Top of lock-stack does not match the unlocked object

[Martin Doerr]

It took me quite a long journey to figure out that C2 OSR goes wrong only in the test "vmTestbase/nsk/jdi/StepEvent" because the interpreter fills the slots in a different order as expected by C2. (Interpreter and C1 don't care about it.)
I've reimplemented the search loop in `monitorenter`, improved comments and cleaned up the related code a bit. The test is passing with this change.

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Commit messages:
 - 8316746: Top of lock-stack does not match the unlocked object

Changes: https://git.openjdk.org/jdk/pull/16406/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16406&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8316746
  Stats: 83 lines in 2 files changed: 16 ins; 34 del; 33 mod
  Patch: https://git.openjdk.org/jdk/pull/16406.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16406/head:pull/16406

PR: https://git.openjdk.org/jdk/pull/16406