The SelectionDAG Isel supports the both version of combines mentioned
below :
```
select Cond, Pow2, 0 --> (zext Cond) << log2(Pow2)
select Cond, 0, Pow2 --> (zext !Cond) << log2(Pow2)
```
The GlobalIsel for now only supports the first one defined in it's
generic combinerHelper.cpp. This patch adds the missing second one.