mips64: fixed problem with Iop's for mips64.

Fixed problem with some Iop's that was discovered while running vbit-test
on mips64. Fixed operations are: Iop_1Uto64, Iop_Left64 and Iop_CmpwNEZ64.


git-svn-id: svn://svn.valgrind.org/vex/trunk@2757 8f6e269a-dfd6-0310-a8e1-e2731360e62c
Enhance ado_treebuild_BB to allow an expression preceding a Put
statement and containing one or more Get expressions to be
substituted in an expression following the Put statement.
That transformation is harmless as long as the guest state areas being
accessed by the Put and Get(s) do not overlap.


git-svn-id: svn://svn.valgrind.org/vex/trunk@2758 8f6e269a-dfd6-0310-a8e1-e2731360e62c