Convert ZEND_ECHO operand to string

[TysonAndre]

And filter out echoes of the empty string (e.g. false/null)

Split out of #5097

[nikic]

These comments don't really make sense in this context anymore.

[nikic]

I'd write this as:

 if (Z_TYPE_P(val) != IS_STRING && zend_optimizer_eval_cast(&zv, IS_STRING, val) == SUCCESS) {
zval_ptr_dtor_nogc(val);
val = &zv;
}
opline->op1.constant = zend_optimizer_add_literal(op_array, val);

Destroying val is technically not necessary because it so happens that the cast can only succeed if the original value wasn't refcounted, but I think this makes the logic of the code more obvious.

[TysonAndre]

Is IS_RESOURCE possible (e.g. STDOUT)? That is refcounted

[nikic]

I believe inside opcache it's not possible.