RFR: 8303564: C2: "Bad graph detected in build_loop_late" after a CMove is wrongly split thru phi

[Roland Westrelin]

The following steps lead to the crash:

- In `testHelper()`, the null and range checks for the `field1[0]`
  load are hoisted out of the counted loop by loop predication
  
- As a result, the `field1[0]` load is also out of loop, control
  dependent on a predicate
  
- pre/main/post loops are created, the main loop is unrolled, the `f`
  value that's stored in `field3` is a Phi that merges the values out
  of the 3 loops.

- the `stop` variable that captures the limit of the loop is
  transformed into a `Phi` that merges 1 and 2.
  
- As a result, the Phi that's stored in `field3` now only merges the
  value of the pre and post loop and is transformed into a `CmoveI`
  that merges 2 values dependent on the `field1[0]` `LoadI` that's
  control dependent on a predicate.
  
- On the next round of loop opts, the `CmoveI` is assigned control
  below the predicate but the `Bool`/`CmpI` for the `CmoveI` is
  assigned control above, right below a `Region` that has a `Phi` that
  is input to the `CmpI`. The reason is this logic: 
  https://github.com/rwestrel/jdk/blob/99f5687eb192b249a4a4533578f56b131fb8f234/src/hotspot/share/opto/loopnode.cpp#L5968
  
- The `CmoveI` is split thru phi because the `Bool`/`CmpI` have
  control right below a `Region`. That shouldn't happen because the
  `CmoveI` itself doesn't have control at the `Region` and is actually
  pinned through the `LoadI` below the `Region`.
  
The fix I propose is to check the control of the `CmoveI` before
proceding with split thru phi.

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Commit messages:
 - fix

Changes: https://git.openjdk.org/jdk/pull/12851/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12851&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303564
  Stats: 82 lines in 2 files changed: 80 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/12851.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12851/head:pull/12851

PR: https://git.openjdk.org/jdk/pull/12851