test(compiler): add signed integer arithmetic operator tests

- Create new test file for signed integer arithmetic operations
- Add test cases for addition and subtraction with negative values
- Verify that non-overflowing results maintain integer type
- Test increment and decrement operations with integer preservation
- Include expected output for all arithmetic operations
- Validate type checking behavior with requireInt function calls

tests/compiler/operator/signed-integer-arithmetic.phpt

@@ -0,0 +1,32 @@
+--TEST--
+signed integer arithmetic keeps non-overflowing results as integers
+--FILE--
+<?php
+
+function requireInt(int $value): void
+{
+    var_dump($value);
+}
+
+function main(): void
+{
+    $values = [-1, 1, 0];
+    $negativeOne = $values[0];
+    $positiveOne = $values[1];
+    $zero = $values[2];
+
+    requireInt($negativeOne + $positiveOne);
+    requireInt($zero - $positiveOne);
+
+    $negativeOne += $positiveOne;
+    requireInt($negativeOne);
+
+    $zero -= $positiveOne;
+    requireInt($zero);
+}
+?>
+--EXPECT--
+int(0)
+int(-1)
+int(0)
+int(-1)