s390x/tcg: Implement VECTOR MERGE (HIGH|LOW)

s390x/tcg: Implement VECTOR MERGE (HIGH|LOW)

We cannot use gvec expansion as source and destination elements are
have different element numbers. So we'll expand using a fancy loop.
Also, we have to take care of overlapping source and destination
registers, therefore use a safe evaluation irder depending on the
operation.

Reviewed-by: Richard Henderson <[email protected]>
Signed-off-by: David Hildenbrand <[email protected]>
Message-Id: <20190307121539[email protected]>
Signed-off-by: Cornelia Huck <[email protected]>