Add test for GH-20880 (#20919)

Closes GH-20880.

ext/opcache/tests/jit/gh20880.phpt

@@ -0,0 +1,27 @@
+--TEST--
+GH-20880 (JIT (tracing): NAN float comparisons incorrectly return true)
+--INI--
+opcache.enable=1
+opcache.enable_cli=1
+--FILE--
+<?php
+function observe(float $value, float $max): float {
+    if ($value > $max) {
+        return $value;
+    }
+    return $max;
+}
+
+
+$max = 0.0;
+for ($i = 0; $i < 100000; $i++) {
+    $max = observe(1.0, $max);
+    $max = observe(3.0, $max);
+}
+
+$max = observe(4.0, $max);
+$max = observe(NAN, $max);
+var_dump($max);
+?>
+--EXPECT--
+float(4)