[lworld] RFR: 8367405: [lworld] C2 compilation fails with empty program detected during loop optimization

[Damon Fenacci]

## Issue
The symptom is that an empty program is detected during loop optimization when running `compiler/stringopts/TestStackedConcatsAppendUncommonTrap.java` with `-XX:-TieredCompilation -XX:+StressUnstableIfTraps`.

## Cause
The cause's origin is string concatenation failing to bail out. More specifically, at the end of `StringConcat::validate_control_flow`, while validating the results, we miss processing a `Phi` node and wrongly let the validation pass. The starting point is this:

<img width="938" height="820" alt="image" src="https://github.com/user-attachments/assets/b7a3ee17-33d7-4ecb-8fb0-fb962a831f04" />

While processing the `Proj` node and going through its outputs, we reach the `CheckCastPP` and add its uses to the worklist (basically only the `Phi` node).

https://github.com/openjdk/valhalla/blob/708b4f92431df90c115dac840fb8194ec3aac3fe/src/hotspot/share/opto/stringopts.cpp#L1138-L1145

Surprisingly we don't add the `CheckCastPP` node itself. 
In one of the next worklist iterations we process the `Phi` node and its uses and none of them make the validation fail.

Side note: `CheckCastPP` is added as additional profiling in Valhalla when parsing `acmp`
https://github.com/openjdk/valhalla/blob/23dc125e5791238a97923108b77c6fc0b59d894f/src/hotspot/share/opto/parse2.cpp#L2098-L2102

On the other hand in mainline `CheckCastPP` is not added and when processing the `Proj`, `Phi` is encountered as one of its uses and the validation fails straight away.

## Fix

Unfortunately this looks yet like another patch in the stacked optimisation's pattern matching "denylist" approach. As mentioned by @danielogh and @robcasloz in the JBS issue, a long-term better approach would be to limit the concatenation to known safe cases.

While waiting for that, a reasonable ad-hoc solution for this Valhalla case is to add a specific `CheckCastPP` case to the validation of uses that also adds itself to the worklist. This will include `CheckCastPP` nodes added by Valhalla's additional profiling (in all fairness this doesn't exclude potential other origins, but the extend and outcome in terms of concatenation optimisation failures should be quite limited).
Note: `CastPP` has to be treated differently: it seems that not adding itself in the worklist was done on purpose for `CastPP` to skip string null check `Phi` nodes among other things.

## Testing

Tests: tier 1-3

-------------

Commit messages:
 - JDK-8367405: simplify
 - JDK-8367405: ad-hoc CheckCastPP handling
 - JDK-8367405: add string concatenation output check to Test7179138_1
 - Merge branch 'lworld' into JDK-8367405
 - JDK-8367405: [lworld] C2 compilation fails with empty program detected during loop optimization

Changes: https://git.openjdk.org/valhalla/pull/1675/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1675&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8367405
  Stats: 5 lines in 2 files changed: 3 ins; 2 del; 0 mod
  Patch: https://git.openjdk.org/valhalla/pull/1675.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1675/head:pull/1675

PR: https://git.openjdk.org/valhalla/pull/1675