RFR: 8346184: C2: assert(has_node(i)) failed during split thru phi

[Roland Westrelin]

The assert fires during split thru phi because a call to `Identity`
returns a new node (a constant null pointer). That happens because a
`Load`, once pushed thru phi, can be constant folded because it loads
from a newly allocated array. `Identity` shouldn't return new
nodes. When split thru phi runs, in this case, `Value` should be the
one returning constant null, not `Identity`. There is logic for that
in `LoadNode::Value` but it's after some other checks that cause
`Value` to return too early.

To fix this, I propose reordering checks in `LoadNode::Value`.

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Commit messages:
 - test
 - fix

Changes: https://git.openjdk.org/jdk/pull/22818/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22818&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8346184
  Stats: 95 lines in 2 files changed: 81 ins; 14 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/22818.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22818/head:pull/22818

PR: https://git.openjdk.org/jdk/pull/22818