RFR: 8353842: C2: Add graph dumps before and after loop opts phase

[Manuel Hässig]

This PR adds graph dumps before and after loop optimizations, but only if the compiled method actually contains loops. This helps to distinguish loop optimizations in IGV and to match loop related nodes like opaque template assertion predicates.

I tested this by compiling a few test methods and looking at the ideal graph. Also, I ran tier1 through tier3 and Oracle internal testing.

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Commit messages:
 - Add before and after loop optimization phases

Changes: https://git.openjdk.org/jdk/pull/24509/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24509&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8353842
  Stats: 13 lines in 3 files changed: 11 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/24509.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24509/head:pull/24509

PR: https://git.openjdk.org/jdk/pull/24509