RFR: 8254887: C2: assert(cl->trip_count() > 0) failed: peeling a fully unrolled loop

[Roland Westrelin]

A loop's trip count is computed to have exact trip count 6. Then:

1- pre/main/post loops are created which brings the trip count from 6
  to 5
2- main loop is unrolled which brings the trip count to 2
3- main loop is peeled: Trip count is 1
4- pre/main/post loops are created again. Trip count of main loop is 0.
5- peeling is attempted again and the assert fires

IdealLoopTree::policy_peeling() doesn't attempt peeling if the trip
count is 1. I propose that IdealLoopTree::policy_range_check() (that
causes the pre/main/post loops insertion the second time) performs the
same check so step 4 doesn't happen.

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Commit messages:
 - test
 - fix

Changes: https://git.openjdk.java.net/jdk/pull/1096/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1096&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254887
  Stats: 63 lines in 2 files changed: 62 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1096.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1096/head:pull/1096

PR: https://git.openjdk.java.net/jdk/pull/1096