RFR: 8255900: x86: Reduce impact when VerifyOops is disabled

[Claes Redestad]

- Don't generate stub _verify_oop_subroutine_entry on startup if -VerifyOops
- Make VerifyOops check in calls to MacroAssembler::verify_oop inlineable. (VerifyOops is a develop flag, so this allows more aggressive code elimination in a few place)
- (trivial pile-on) remove unused set_word_if_not_zero

Slightly improves static footprint, gets rid of a stub and reduce jitter during jitting in product builds.

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Commit messages:
 - Fix copy-paste error
 - add checked variants of _verify_oop to avoid calls in product builds
 - Merge branch 'master' into verify_oop_stub
 - Cleanup verify_oop

Changes: https://git.openjdk.java.net/jdk/pull/1058/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1058&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8255900
  Stats: 27 lines in 4 files changed: 13 ins; 9 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1058.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1058/head:pull/1058

PR: https://git.openjdk.java.net/jdk/pull/1058