RFR: 8267928: Loop predicate gets inexact loop limit before PhaseIdealLoop::rc_predicate

[Yi Yang]

Loop predicate gets inexact loop limit(LoopLimitNode) from exact_limit and does unnecessary overflow checking when generating lower bound test(rc_predicate). The reason is rather straightforward: exact_limit fails to see a HasExactTripCount flag since it would be set after performing loop predicate(iteration_split).

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Commit messages:
 - compute_trip_count

Changes: https://git.openjdk.java.net/jdk/pull/4247/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4247&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8267928
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4247.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4247/head:pull/4247

PR: https://git.openjdk.java.net/jdk/pull/4247