minor #3896 Compile Elvis operator into Elvis operator ?: (GromNaN)

This PR was merged into the 3.x branch.

Discussion
----------

Compile Elvis operator into Elvis operator `?:`

When using ternary operator without "then" part, the "condition" part is evaluated twice, which is inconsistent with how PHP works.
The Twig template `A ?: B` is currently compiled as `A ? A : B` in PHP. This PR change it to `A ?: B`.

If `A` is a complex expression, it improves performance to only execute the expression once.
If `A` is has a side effect (like updating a variable), the expression being executed twice could result in a bug. ([example in PHP](https://3v4l.org/LWZLR))

Example with ``@WebProfiler`/Collector/form.html.twig` [line 9](https://github.com/symfony/symfony/blob/e6d1ed4edb5ae197ec7d25ddaf64cfa456229504/src/Symfony/Bundle/WebProfilerBundle/Resources/views/Collector/form.html.twig#L9) (see deep [diff](https://www.diffchecker.com/jOm3dE13/))

```twig
{{ collector.data.nb_errors ?: collector.data.forms|length }}
```

Previous compilation:
```php
echo twig_escape_filter($this->env, (((isset($context["error_count"]) || array_key_exists("error_count", $context) ? $context["error_count"] : (function () { throw new RuntimeError('Variable "error_count" does not exist.', 9, $this->source); })())) ? ((isset($context["error_count"]) || array_key_exists("error_count", $context) ? $context["error_count"] : (function () { throw new RuntimeError('Variable "error_count" does not exist.', 9, $this->source); })())) : (twig_get_attribute($this->env, $this->source, (isset($context["collector"]) || array_key_exists("collector", $context) ? $context["collector"] : (function () { throw new RuntimeError('Variable "collector" does not exist.', 9, $this->source); })()), "countDefines", [], "any", false, false, false, 9))), "html", null, true);
```

After this change:
```php
echo twig_escape_filter($this->env, ((isset($context["error_count"]) || array_key_exists("error_count", $context) ? $context["error_count"] : (function () { throw new RuntimeError('Variable "error_count" does not exist.', 9, $this->source); })()) ?: twig_get_attribute($this->env, $this->source, (isset($context["collector"]) || array_key_exists("collector", $context) ? $context["collector"] : (function () { throw new RuntimeError('Variable "collector" does not exist.', 9, $this->source); })()), "countDefines", [], "any", false, false, false, 9)), "html", null, true);
```

Commits
-------

fb0d7495 Compile Elvis operator with Elvis operator

src/ExpressionParser.php

@@ -183,11 +183,14 @@ class ExpressionParser
             if (!$this->parser->getStream()->nextIf(/* Token::PUNCTUATION_TYPE */ 9, ':')) {
                 $expr2 = $this->parseExpression();
                 if ($this->parser->getStream()->nextIf(/* Token::PUNCTUATION_TYPE */ 9, ':')) {
+                    // Ternary operator (expr ? expr2 : expr3)
                     $expr3 = $this->parseExpression();
                 } else {
+                    // Ternary without else (expr ? expr2)
                     $expr3 = new ConstantExpression('', $this->parser->getCurrentToken()->getLine());
                 }
             } else {
+                // Ternary without then (expr ?: expr3)
                 $expr2 = $expr;
                 $expr3 = $this->parseExpression();
             }

src/Node/Expression/ConditionalExpression.php

@@ -23,14 +23,23 @@ class ConditionalExpression extends AbstractExpression
 
     public function compile(Compiler $compiler): void
     {
-        $compiler
-            ->raw('((')
-            ->subcompile($this->getNode('expr1'))
-            ->raw(') ? (')
-            ->subcompile($this->getNode('expr2'))
-            ->raw(') : (')
-            ->subcompile($this->getNode('expr3'))
-            ->raw('))')
-        ;
+        // Ternary with no then uses Elvis operator
+        if ($this->getNode('expr1') === $this->getNode('expr2')) {
+            $compiler
+                ->raw('((')
+                ->subcompile($this->getNode('expr1'))
+                ->raw(') ?: (')
+                ->subcompile($this->getNode('expr3'))
+                ->raw('))');
+        } else {
+            $compiler
+                ->raw('((')
+                ->subcompile($this->getNode('expr1'))
+                ->raw(') ? (')
+                ->subcompile($this->getNode('expr2'))
+                ->raw(') : (')
+                ->subcompile($this->getNode('expr3'))
+                ->raw('))');
+        }
     }
 }