Fix unpacking from a single vector.

Both vector arguments were being used in a punpckl instruction, while
the shuffle mask repeats elements from just the first vector.

BUG=swiftshader:15

Change-Id: I8e29c252ee4957692c4949e724ae67253b423e89
Reviewed-on: https://chromium-review.googlesource.com/399419
Reviewed-by: Jim Stichnoth <stichnot@chromium.org>
Tested-by: Nicolas Capens <nicolascapens@google.com>