[jdk20] RFR: 8298176: remove OpaqueZeroTripGuardPostLoop once main-loop disappears [v2]

[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.

Emanuel Peter has updated the pull request incrementally with one additional commit since the last revision:

  remove swap files, how did I ever commit them?

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Changes:
  - all: https://git.openjdk.org/jdk20/pull/22/files
  - new: https://git.openjdk.org/jdk20/pull/22/files/1dc52234..b83c995e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk20&pr=22&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk20&pr=22&range=00-01

  Stats: 0 lines in 2 files changed: 0 ins; 0 del; 0 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