[lworld] RFR: 8336003: [lworld] TestLWorld::test151 triggers "Should have been buffered" assert [v3]

[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.

Quan Anh Mai has updated the pull request incrementally with one additional commit since the last revision:

  use Unique_Node_List

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Changes:
  - all: https://git.openjdk.org/valhalla/pull/1489/files
  - new: https://git.openjdk.org/valhalla/pull/1489/files/67f5c154..c5071cdb

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=valhalla&pr=1489&range=02
 - incr: https://webrevs.openjdk.org/?repo=valhalla&pr=1489&range=01-02

  Stats: 7 lines in 2 files changed: 0 ins; 1 del; 6 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