[MachineCopyPropagation] Remove redundant copies after TailDup via machine-cp

Summary:
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

llvm-svn: 371359