[llvm] [PowerPC] fixed issue "Failure to optimize (x == 0) ? 0xFF : 0 to addic+subfe instead of cntlzw+srwi+neg" (PR #190606)

[Lei Huang via llvm-commits]

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lei137 wrote:

same as before. Early exit
```
 if (CC != ISD::SETEQ || (!isNullConstant(LHS) && !isNullConstant(RHS)))
    return SDValue();
```

https://github.com/llvm/llvm-project/pull/190606