Use `G_ANYEXT` instead of `G_ZEXT` when widening the source of
`G_CTLZ_ZERO_UNDEF`. The extended upper bits are immediately shifted out
by the subsequent left-shift, so zero-extending is unnecessarily
constraining.
Before:
```
%wide = G_ZEXT %src
%shifted = G_SHL %wide, sizeDiff
%result = G_CTLZ_ZERO_UNDEF %shifted
```
After:
```
%wide = G_ANYEXT %src
%shifted = G_SHL %wide, sizeDiff
%result = G_CTLZ_ZERO_UNDEF %shifted
```
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