Add support for null coalesce assign operator

(cherry picked from commit af90320)

Author: thegallagher

lib/PHPCfg/Parser.php

@@ -836,6 +836,7 @@ protected function parseExprNode($expr)
                 'Expr_AssignOp_BitwiseAnd' => Op\Expr\BinaryOp\BitwiseAnd::class,
                 'Expr_AssignOp_BitwiseOr' => Op\Expr\BinaryOp\BitwiseOr::class,
                 'Expr_AssignOp_BitwiseXor' => Op\Expr\BinaryOp\BitwiseXor::class,
+                'Expr_AssignOp_Coalesce' => Op\Expr\BinaryOp\Coalesce::class,
                 'Expr_AssignOp_Concat' => Op\Expr\BinaryOp\Concat::class,
                 'Expr_AssignOp_Div' => Op\Expr\BinaryOp\Div::class,
                 'Expr_AssignOp_Minus' => Op\Expr\BinaryOp\Minus::class,

test/code/assign_coalesce.test

@@ -0,0 +1,17 @@
+<?php
+
+$x ??= a();
+-----
+Block#1
+    Expr_FuncCall
+        name: LITERAL('a')
+        result: Var#1
+    Expr_BinaryOp_Coalesce
+        left: Var#2<$x>
+        right: Var#1
+        result: Var#3
+    Expr_Assign
+        var: Var#4<$x>
+        expr: Var#3
+        result: Var#5
+    Terminal_Return