[lworld] RFR: 8336003: [lworld] TestLWorld::test151 triggers "Should have been buffered" assert

[Quan Anh Mai]

Hi,

The issue here is that the IGVN cannot deduce that the `IsBuffered` input is a constant 1. This is because after the `InlineTypeNode` is pushed down through a `Phi` cluster, the `IsBuffered` input is a `Phi` cluster with loop phis, which makes it difficult for the IGVN to see through. A solution is to do another round of CCP after loop opts, but it is rather a big hammer. In this PR, I propose to make it possible for a `PhiNode` to look through its `Phi` inputs to discover the unique non-phi input of a `Phi` cluster. The test still fails but it seems more like an expected behaviour rather than an issue this time, I have modified the test a little bit for it to pass.

Please take a look and share your thoughts, thanks very much.

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Commit messages:
 - Optimize the case when all inputs into a Phi cluster is the same node

Changes: https://git.openjdk.org/valhalla/pull/1489/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1489&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8336003
  Stats: 47 lines in 3 files changed: 43 ins; 3 del; 1 mod
  Patch: https://git.openjdk.org/valhalla/pull/1489.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1489/head:pull/1489

PR: https://git.openjdk.org/valhalla/pull/1489