target/s390x: Fix the "ignored match" case in VSTRS

target/s390x: Fix the "ignored match" case in VSTRS

Currently the emulation of VSTRS recognizes partial matches in presence
of \0 in the haystack, which, according to PoP, is not correct:

    If the ZS flag is one and a zero byte was detected
    in the second operand, then there can not be a
    partial match ...

Add a check for this. While at it, fold a number of explicitly handled
special cases into the generic logic.

Cc: qemu-stable@nongnu.org
Reported-by: Claudio Fontana <cfontana@suse.de>
Closes: https://lists.gnu.org/archive/html/qemu-devel/2023-08/msg00633.html
Fixes: 1d706f314191 ("target/s390x: vxeh2: vector string search")
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Message-Id: <20230804233748.218935-3-iii@linux.ibm.com>
Tested-by: Claudio Fontana <cfontana@suse.de>
Acked-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>