[false negative] DoubleCheckedLocking: not reported when the outer null check is written as !(x != null)

**Versions:** Error Prone `error_prone_core` 2.41.0, JDK 25, invoked via `javac` (CLI).

**Description**

`DoubleCheckedLocking` matches the guard by its syntactic form. Rewriting the outer null check from `baz == null` to the logically-equivalent `!(baz != null)` makes the check stop firing, even though the double-checked locking on the non-volatile field — and its unsafe publication — is entirely unchanged. The two programs are behaviourally identical.

**Reproducer (reported):**

```java
public class DclSeed {
Object baz;

Object bar() {
if (baz == null) { // reported here (line 5)
synchronized (this) {
if (baz == null) {
baz = new Object();
}
}
}
return baz;
}
}
```

**Reproducer (NOT reported — the only change is the outer guard):**

```java
public class DclVariant {
Object baz;

Object bar() {
if (!(baz != null)) { // equivalent to `baz == null`; no diagnostic
synchronized (this) {
if (baz == null) {
baz = new Object();
}
}
}
return baz;
}
}
```

**How to run (the `-J--add-exports`/`--add-opens` and `-XDcompilePolicy=simple --should-stop=ifError=FLOW` flags are required for Error Prone on a modern JDK; without them the plugin does not run):**

```
javac \
-J--add-exports=jdk.compiler/com.sun.tools.javac.api=ALL-UNNAMED \
-J--add-exports=jdk.compiler/com.sun.tools.javac.file=ALL-UNNAMED \
-J--add-exports=jdk.compiler/com.sun.tools.javac.main=ALL-UNNAMED \
-J--add-exports=jdk.compiler/com.sun.tools.javac.model=ALL-UNNAMED \
-J--add-exports=jdk.compiler/com.sun.tools.javac.parser=ALL-UNNAMED \
-J--add-exports=jdk.compiler/com.sun.tools.javac.processing=ALL-UNNAMED \
-J--add-exports=jdk.compiler/com.sun.tools.javac.tree=ALL-UNNAMED \
-J--add-exports=jdk.compiler/com.sun.tools.javac.util=ALL-UNNAMED \
-J--add-opens=jdk.compiler/com.sun.tools.javac.code=ALL-UNNAMED \
-J--add-opens=jdk.compiler/com.sun.tools.javac.comp=ALL-UNNAMED \
-XDcompilePolicy=simple --should-stop=ifError=FLOW \
-processorpath error_prone_core-2.41.0-with-dependencies.jar -Xplugin:ErrorProne \
-d out DclSeed.java
# DclSeed.java:5: warning: [DoubleCheckedLocking] Double-checked locking on non-volatile fields is unsafe
# (running the same command on DclVariant.java produces no DoubleCheckedLocking diagnostic)
```

**Expected:** Error Prone should emit `[DoubleCheckedLocking]` on `DclVariant.java` line 5, but does not.

Start with the DclSeed.java and DclVariant.java reproducers and compare how the outer null guards are recognized. Run the supplied javac/Error Prone command, confirming the diagnostic on DclSeed.java and the missing diagnostic on DclVariant.java. Done means the equivalent `!(baz != null)` guard also emits DoubleCheckedLocking on line 5.

Written by the indexing model from the issue text.