Support the cu14 insn. That insn is very much like cu12 except the
converted value is always 4 byte wide. The only other difference is
the encoding of a 4-byte UTF-8 character.
Some code refactoring does the trick.
Part of fixing #289839.


git-svn-id: svn://svn.valgrind.org/vex/trunk@2459 8f6e269a-dfd6-0310-a8e1-e2731360e62c
3 files changed