Add function s390_isel_amode_b12_b20 to compile an expression into an
amode that is either S390_AMODE_B12 or S390_AMODE_B20. This is needed
for compare-and-swap insns. As we're currently not generating amodes
using an index register, there was never a problem. 
This change future-proofs the code.
Also add a few more asserts for amodes in the s390_insns supporting
translation chaining.
Fixes BZ #269360.


git-svn-id: svn://svn.valgrind.org/vex@3000 8f6e269a-dfd6-0310-a8e1-e2731360e62c