Unused local elimination may cause side effects to vanish

Environment:
ECJ Version:

```
Eclipse Compiler for Java(TM) v20250514-1000, 3.42.0, Copyright IBM Corp 2000, 2020. All rights reserved.
```
Java Version:

```
java 21.0.7 2025-04-15 LTS
Java(TM) SE Runtime Environment (build 21.0.7+8-LTS-245)
Java HotSpot(TM) 64-Bit Server VM (build 21.0.7+8-LTS-245, mixed mode, sharing)
```
Javac Version:

```
javac 21.0.7
```
Description:
The following test program exhibits different runtime behaviors when compiled with different compilers (ECJ and javac):

```java
public class Test {
public static void main(String[] args) {
int result = (Integer) null;
}
}
```

When compiling this code with javac and running it, the program throws a `java.lang.NullPointerException`.

However, when compiling with ECJ and running the program, no exception is thrown. This is incorrect behavior as it is trying to unbox a null Integer value to a primitive int.

Another case that shows the problem, this time missing ArithmeticException:

```
class Test {
static int val = 0;

public static void main(String[] argv) {
val = 50;
for (int i = 0; i < 10; i++) {
for (int j = 0; j < 10; j++) {
byte b = (byte) i;
int result = b * j;
result <<= 2;
result /= j;
}
}
}
}
```

Steps to Reproduce:
1. Compile the test program using javac, and run it. The program throws a NullPointerException:
```bash
javac Test.java
java Test
Exception in thread "main" java.lang.NullPointerException: Cannot invoke "java.lang.Integer.intValue()" because "null" is null
at Test.main(Test.java:3)
```

2. Compile the test program using ECJ, and run it. The program executes successfully without throwing any exception:
```bash
java -jar ecj-4.36M3.jar --release 21 -d . Test.java
java Test
```

Expected behavior: The program compiled with ECJ should throw a NullPointerException.
Actual behavior: The program compiled with ECJ executes successfully without throwing any exception.