RFR: 8356708: C2: loop strip mining expansion doesn't take sunk stores into account

[Roland Westrelin]

`test1()` has a counted loop with a `Store` to `field`. That `Store`
is sunk out of loop. When the `OuterStripMinedLoop` is expanded, only
`Phi`s that exist at the inner loop are added to the outer
loop. There's no `Phi` for the slice of the sunk `Store` (because
there's no `Store` left in the inner loop) so no `Phi` is added for
that slice to the outer loop. As a result, there's a missing anti
dependency for `Load` of `field` that's before the loop and it can be
scheduled inside the outer strip mined loop which is incorrect.

`test2()` is the same as `test1()` but with a chain of 2 `Store`s.

`test3()` is another variant where a `Store` is left in the inner loop
after one is sunk out of it so the inner loop still has a `Phi`. As a
result, the outer loop also gets a `Phi` but it's incorrectly wired as
the sunk `Store` should be the input along the backedge but is
not. That one doesn't cause any failure AFAICT.

The fix I propose is some extra logic at expansion of the
`OuterStripMinedLoop` to handle these corner cases.

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Commit messages:
 - test & fix

Changes: https://git.openjdk.org/jdk/pull/25717/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25717&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8356708
  Stats: 188 lines in 3 files changed: 180 ins; 0 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/25717.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25717/head:pull/25717

PR: https://git.openjdk.org/jdk/pull/25717