s390: Support insns to convert between DFP values and signed/unsigned
integers. Patch by Maran Pakkirisamy (maranp@linux.vnet.ibm.com).
Part of fixing BZ 307113.


git-svn-id: svn://svn.valgrind.org/vex/trunk@2632 8f6e269a-dfd6-0310-a8e1-e2731360e62c
Fix a comment and add one.


git-svn-id: svn://svn.valgrind.org/vex/trunk@2633 8f6e269a-dfd6-0310-a8e1-e2731360e62c