Optimize destructuring of array literals to elide intermediate array

[TimWolla]

Description

The following code:

<?php

$a = 1;
$b = 2;

[$a, $b] = [$b, $a];

var_dump($a, $b);

currently compiles as:

$_main:
     ; (lines=14, args=0, vars=2, tmps=2)
     ; (after optimizer)
     ; test6.php:1-9
0000 ASSIGN CV0($a) int(1)
0001 ASSIGN CV1($b) int(2)
0002 T2 = INIT_ARRAY 2 (packed) CV1($b) NEXT
0003 T2 = ADD_ARRAY_ELEMENT CV0($a) NEXT
0004 T3 = FETCH_LIST_R T2 int(0)
0005 ASSIGN CV0($a) T3
0006 T3 = FETCH_LIST_R T2 int(1)
0007 ASSIGN CV1($b) T3
0008 FREE T2
0009 INIT_FCALL 2 112 string("var_dump")
0010 SEND_VAR CV0($a) 1
0011 SEND_VAR CV1($b) 2
0012 DO_ICALL
0013 RETURN int(1)
LIVE RANGES:
     2: 0003 - 0008 (tmp/var)

it should be optimized to something like:

0000 ASSIGN CV0($a) int(1)
0001 ASSIGN CV1($b) int(2)
0002 T1 = CV0($a)
0003 T2 = CV0($b)
0004 ASSIGN CV0($a) T2
0005 ASSIGN CV1($b) T1
0006 INIT_FCALL 2 112 string("var_dump")
0007 SEND_VAR CV0($a) 1
0008 SEND_VAR CV1($b) 2
0009 DO_ICALL
0010 RETURN int(1)