RFR: 8334724: C2: remove PhaseIdealLoop::cast_incr_before_loop()

[Roland Westrelin]

I propose removing `PhaseIdealLoop::cast_incr_before_loop()` and the
`CastII` nodes that it adds at counted loop heads.

They were added to prevent nodes to float above the zero trip guard
when the backedge of a counted loop is removed. In particular, when a
range check is hoisted by predication, pre/main/post loops are created
and if one of the main or post loops lose its backedge, an array load
that's control dependent on a predicate above the pre loop could float
above the zero trip guard of the main or post loop. That can no longer
happen AFAICT with changes related to assert predicates. The array
load is now updated to have a control dependency that's below the zero
trip guard.

The reason I'm revisiting this is that I noticed that
`PhaseIdealLoop::cast_incr_before_loop()` has a bug. When it adds the
`CastII`, it looks for the loop phi and picks input 1 of the phi it
finds as input to the `CastII`. To find the loop phi, it starts from
the loop incremement and loop for a use that's a phi and has the loop
head as control. It never checks that the phi it finds is the loop
phi. There can be more than one phi as uses of the increment at the
loop head and it can pick the wrong one. I tried to write a test case
where this would cause a bug but couldn't actually find any use for
the `CastII` anymore.

In my testing, the only issue when the `CastII` are not added is that
some IR tests for vectorization fails:

compiler/vectorization/TestPopulateIndex.java
compiler/vectorization/runner/ArrayShiftOpTest.java
compiler/vectorization/runner/LoopArrayIndexComputeTest.java

because removing the `CastII` causes split if to occur with some nodes
that take the loop phi as input. That then causes pattern matching
during superword to break. I added logic to prevent split if for those
cases.

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Commit messages:
 - fix

Changes: https://git.openjdk.org/jdk/pull/19831/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19831&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8334724
  Stats: 61 lines in 3 files changed: 27 ins; 28 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/19831.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19831/head:pull/19831

PR: https://git.openjdk.org/jdk/pull/19831