Post splat variables from MultipleAssignment should be scanned backwards.

Fixes rubinius/rubinius#3030.

Before:
`[a, b, c, d] => [[1, 2], 5, 4, 3]`

The generated bytecode pops from the array's tail and variables are assigned in this order: b, c, d.

After:
`[a, b, c, d] => [[1, 2], 3, 4, 5]`

The generated bytecode pops from the array's tail and variables are assigned in the correct (backwards) order: d, c, b.