mips: implement Iop_8Uto64 in iselInt64Expr

Iop_8Uto64 was missing in iselInt64Expr.
The change should resolve Bugzilla #319505.


git-svn-id: svn://svn.valgrind.org/vex/trunk@2729 8f6e269a-dfd6-0310-a8e1-e2731360e62c