RFR: 8316746: Top of lock-stack does not match the unlocked object [v6]

[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.

Martin Doerr has updated the pull request incrementally with one additional commit since the last revision:

  Shorter code: Use Rfree_slot as monitor.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/16406/files
  - new: https://git.openjdk.org/jdk/pull/16406/files/56209f27..0c1a9343

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16406&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16406&range=04-05

  Stats: 7 lines in 1 file changed: 0 ins; 3 del; 4 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