RFR: 8340102: Move assert-only loop in OopMapSort::sort under debug macro

[Aleksey Shipilev]

Found this papercut when looking at Leyden perf runs.

In OopMapSort::sort, there is a loop that apparently is only there for asserts. At least GCC 11.4 apparently not smart enough to eliminate the whole loop in release builds, probably because iterator reads things from the stream. Wrapping the loop with #ifdef ASSERT saves about 144 bytes in code stream.

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/20992/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20992&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340102
  Stats: 4 lines in 1 file changed: 3 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/20992.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20992/head:pull/20992

PR: https://git.openjdk.org/jdk/pull/20992