RFR: 8281553: Ensure we only require liveness from mach-nodes with barriers [v2]

[Emanuel Peter]

> Adding NULL check. This also ensures we are not returning a NULL reference.
> 
> Ran tests to ensure assert is never triggered.

Emanuel Peter has updated the pull request incrementally with one additional commit since the last revision:

  fixing whitespaces

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/7407/files
  - new: https://git.openjdk.java.net/jdk/pull/7407/files/1c889678..b005c5b0

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=7407&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=7407&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7407.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7407/head:pull/7407

PR: https://git.openjdk.java.net/jdk/pull/7407