target/s390x: Fix LCBB overwriting the top 32 bits

target/s390x: Fix LCBB overwriting the top 32 bits

LCBB is supposed to overwrite only the bottom 32 bits, but QEMU
erroneously overwrites the entire register.

Fixes: 6d9303322ed9 ("s390x/tcg: Implement LOAD COUNT TO BLOCK BOUNDARY")
Cc: qemu-stable@nongnu.org
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Message-Id: <20230526181240.1425579-2-iii@linux.ibm.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>