RFR: 8300127: Serial: Remove unnecessary from-space iteration in DefNewGeneration::oop_since_save_marks_iterate [v2]

[Albert Mingkun Yang]

> Converting no-ops to assertions in young-gen obj iteration.
> 
> Test: hotspot_gc

Albert Mingkun Yang has updated the pull request incrementally with one additional commit since the last revision:

  review

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Changes:
  - all: https://git.openjdk.org/jdk/pull/11995/files
  - new: https://git.openjdk.org/jdk/pull/11995/files/4a0ab411..70f09001

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11995&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11995&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/11995.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11995/head:pull/11995

PR: https://git.openjdk.org/jdk/pull/11995