This commit makes the assembler frontend reorder the operands so that
they are in the order the backend expects. The index should be first
for consistency with the other vector instructions.

Before this commit no operand order would have been accepted so this
isn't a breaking change.

Change-Id: I188d57eeb338d27fa1fa6845de0d6d1521b7a6c3
Reviewed-on: https://go-review.googlesource.com/29855
Run-TryBot: Michael Munday <munday@ca.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bill O'Farrell <billotosyr@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>