Fix %rip-relative addressing, which uses the address of the _next_

Fix %rip-relative addressing, which uses the address of the _next_
instruction, not the current one.  Grrr.  This is inconvenient since
it means we sometimes need to know how long an insn is before it is
decoded, and that information is only available after is is decoded.

Currently we make guesses about insn length when a %rip-relative
address appears.  If such a guess is made, it is then checked for
correctness after the decode, so at least all errors should get picked
up.

git-svn-id: svn://svn.valgrind.org/vex/trunk@864