Before this patch, redundant COPY couldn't be removed for the following
case:
```
$R0 = OP ...
... // Read of %R0
$R1 = COPY killed $R0
```
This patch adds support for tracking the users of the source register
during backward propagation, so that we can remove the redundant COPY in
the above case and optimize it to:
```
$R1 = OP ...
... // Replace all uses of %R0 with $R1
```
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