[lworld+vector] RFR: 8317699: [lworld+vector] Fix Vector API tests crash with "assert(vbox->is_Phi()) failed: should be phi"

[Xiaohong Gong]

Currently several jtreg tests under `test/jdk/jdk/jdk/incubator/vector` crash with following error:


A fatal error has been detected by the Java Runtime Environment:

Internal Error (valhalla/src/hotspot/share/opto/vector.cpp:257), pid=2737779, tid=2737796
assert(false) failed: should be phi


It happens when expanding a `VectorBoxNode` which has following inputs:

                        VectorBoxAllocate
                               |
                               |
       VectorBoxAllocate    [2 Proj]
               |             /   |
            [1 Proj]       /     |
               \         /       |
                 \     /        /
                   \ /        /
                 [2 Phi]    /
                     \    /
                       \ |
                     [1 Phi]     InlineType
                         \      /
                           \  /
                         VectorBox

The compiler will visit and expand all the inputs of the `PhiNode`, and mark them visited. If the input has been visited, it just returns without any action. And the compiler assumes the revisited node should be a PhiNode. In above graph, the first `Phi` (i.e. `[1 Phi]`) has the same input `[2 Proj]` with the second `Phi` (i.e. `[2 Phi]`). So `[2 Proj]` will be visited twice. At the second time it is visited, the assertion fails since it is not a Phi. The case is reasonable in java and the assertion can be removed.

Removing the assertion can fix the existing issue and no new issues are involved.

Jdk mainline also has the same issue, but I prefer fixing it here temporarily with following reasons:
 1) Several jtreg tests crash on this branch while pass on jdk mainline. This blocks our testing and further development.
 2) We can have more testing with this fix on this branch. And port it to jdk mainline once we think it's a mature/safe change.

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

Commit messages:
 - 8317699: [lworld+vector] Fix Vector API tests crash with "assert(vbox->is_Phi()) failed: should be phi"

Changes: https://git.openjdk.org/valhalla/pull/937/files
 Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=937&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8317699
  Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/valhalla/pull/937.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/937/head:pull/937

PR: https://git.openjdk.org/valhalla/pull/937