Compile Elvis operator into Elvis operator ?:

[GromNaN]

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)

Example with @WebProfiler/Collector/form.html.twig line 9 (see deep diff)

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

Previous compilation:

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:

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);

[fabpot]

Thank you @GromNaN.