[MachineCopyPropagation] Remove redundant copies after TailDup via machine-cp
After tailduplication, we have redundant copies. We can remove these
copies in machine-cp if it's safe to, i.e.
$reg0 = OP ... ... <<< No read or clobber of $reg0 and $reg1 $reg1 = COPY $reg0 <<< $reg0 is killed ... <RET>
will be transformed to
$reg1 = OP ... ... <RET>
Differential Revision: https://reviews.llvm.org/D65267