s390x/tcg: Fix BRASL with a large negative offset

When RI2 is 0x80000000, qemu enters an infinite loop instead of jumping
backwards. Fix by adding a missing cast, like in in2_ri2().

Fixes: 8ac33cdb ("Convert BRANCH AND SAVE")
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Message-Id: <20220314104232.675863-2-iii@linux.ibm.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>