RFR: 8253524: C2: Refactor code that clones predicates during loop unswitching [v4]

[Roland Westrelin]

> During unswitching, PhaseIdealLoop::create_slow_version_of_loop()
> calls PhaseIdealLoop::clone_predicates_to_unswitched_loop() twice, one
> for each loops, to clone some predicates above each loop. That code is
> fragile as it (implicitly) requires the fast loop to be processed
> first. I propose calling
> PhaseIdealLoop::clone_predicates_to_unswitched_loop() a single time
> and have it handle both loops in a single pass.

Roland Westrelin has refreshed the contents of this pull request, and previous commits have been removed. The
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  extra space

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/317/files
  - new: https://git.openjdk.java.net/jdk/pull/317/files/93f33e9b..33fb2fe2

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=317&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=317&range=02-03

  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/317.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/317/head:pull/317

PR: https://git.openjdk.java.net/jdk/pull/317