RFR: 8262721: Add Tests to verify single iteration loops are properly optimized

[Roland Westrelin]

That came up in previous:

https://github.com/openjdk/jdk/pull/2529#discussion_r583054321

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Commit messages:
 - tests

Changes: https://git.openjdk.java.net/jdk/pull/7368/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7368&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8262721
  Stats: 139 lines in 1 file changed: 139 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7368.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7368/head:pull/7368

PR: https://git.openjdk.java.net/jdk/pull/7368