RFR: 8264480: Unreachable code in nmethod.cpp inside #ifdef DEBUG

[Robbin Ehn]

Fixed ifdef to use correct define.

Stress tested locally, running t1-5.

-------------

Commit messages:
 - Fixed ifdef

Changes: https://git.openjdk.java.net/jdk/pull/3517/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3517&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264480
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3517.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3517/head:pull/3517

PR: https://git.openjdk.java.net/jdk/pull/3517