The element index needs to be placed in From3. Before this CL it
was impossible to write a VSTE instruction that could be
successfully parsed, so this won't affect existing assembly code.

Fixes #18075.

Change-Id: I5b71be4c6632b1d5a30820a529122f96fd1bc864
Reviewed-on: https://go-review.googlesource.com/33584
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: Ian Lance Taylor <iant@golang.org>