RFR: 8281732: add assert for non-NULL assumption for return of unique_ctrl_out [v3]

[Emanuel Peter]

> unique_ctrl_out was used in contexts where NULL may be a valid return, and is also used in contexts where NULL is not expected.
> 
> I improved the code by having two functions instead:
> unique_ctrl_out_or_null: return the unique control out, or NULL if there is no or more than one control out.
> unique_ctrl_out: return the unique control out, assert if there is no or more than one control out.
> 
> This makes implicit assumptions explicit and also validates them in the future.
> 
> I changed the usage to unique_ctrl_out_or_null where NULL is among the expected return values.
> 
> Ran tests to verify that the assert never triggers.

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

  Require non-Null in some more cases

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/7462/files
  - new: https://git.openjdk.java.net/jdk/pull/7462/files/d2efb501..3beb58ce

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=7462&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=7462&range=01-02

  Stats: 5 lines in 3 files changed: 0 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7462.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7462/head:pull/7462

PR: https://git.openjdk.java.net/jdk/pull/7462