[jdk20] RFR: 8298176: remove OpaqueZeroTripGuardPostLoop once main-loop disappears

[Emanuel Peter]

We recently removed `Opaque2` nodes in [JDK-8294540](https://bugs.openjdk.org/browse/JDK-8294540). `Opaque2` nodes prevented some optimizations during loop-opts. The original idea was to prevent the use of both the un-incremented and incremented value of a loop phi after the loop, to reduce register pressure. But `Opaque2` also had the effect that the limit of the loop would not be optimized, which meant that the iv-value (entry value of phi) in post loop would never collapse (either to constant or TOP), but always remain a range. Now that `Opaque2` is gone, it can happen that when the main-loop disappears, the limit collapses. The zero-trip guard of the post-loop would be false, but does not collapse because of the `OpaqueZeroTripGuard`. The post-loop can half-collapse, leaving an inconsistent graph below the zero-trip guard if.

**Solution**
Have `OpaqueZeroTripGuardMainLoop` for main loop zero-trip guard, and `OpaqueZeroTripGuardPostLoop` for post-loop zero trip guard. Let `OpaqueZeroTripGuardPostLoop` remove itself once it cannot find the main-loop above it. We have these opaque nodes there to prevent collapsing of the zero-trip guards as long as the limits may still change, but after the main-loop is removed, no unrolling is done anymore, so the limit of the post-loop cannot change anymore, hence it is safe to remove the opaque node there.

An alternative approach was to let the main-loop remove the opaque node of the post-loop's zero-trip guard. But that does not work reliably, as the main-loop may get removed during PhaseCCP, and the main-loop is simply removed as "useless". Hence the LoopNode of the main-loop does not have a chance to detect its death during IGVN.

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Commit messages:
 - tab to whitespace
 - code style improvements
 - 8298176: remove OpaqueZeroTripGuardPostLoop once main-loop disappears

Changes: https://git.openjdk.org/jdk20/pull/22/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk20&pr=22&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8298176
  Stats: 293 lines in 13 files changed: 268 ins; 5 del; 20 mod
  Patch: https://git.openjdk.org/jdk20/pull/22.diff
  Fetch: git fetch https://git.openjdk.org/jdk20 pull/22/head:pull/22

PR: https://git.openjdk.org/jdk20/pull/22