Prevent optimization of huge functions.

ext/opcache/Optimizer/zend_ssa.c

@@ -847,6 +847,11 @@ int zend_build_ssa(zend_arena **arena, const zend_script *script, const zend_op_
 	ALLOCA_FLAG(dfg_use_heap)
 	ALLOCA_FLAG(var_use_heap)
 
+	if ((blocks_count * (op_array->last_var + op_array->T)) > 4 * 1024 * 1024) {
+	    /* Don't buld SSA for very big functions */
+		return FAILURE;
+	}
+
 	ssa->rt_constants = (build_flags & ZEND_RT_CONSTANTS);
 	ssa_blocks = zend_arena_calloc(arena, blocks_count, sizeof(zend_ssa_block));
 	if (!ssa_blocks) {