cmd/asm, cmd/internal/obj/s390x: fix encoding of VREPI{H,F,G}

Also adds tests for all missing VRI-a instructions (which may be
affected by this change).

Fixes #18749.

Change-Id: I48249dda626f32555da9ab58659e2e140de6504a
Reviewed-on: https://go-review.googlesource.com/35561


Run-TryBot: Michael Munday <munday@ca.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>