[llvm] [PowerPC] improve performance on the isNan and !isNan function in case of -ffp-model=strict (PR #204170)

[zhijian lin via llvm-commits]

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diggerlin wrote:

if there is  VT == MVT::f128 ,it will go to above code 

```
if (VT == MVT::ppcf128) {
    // The higher part determines the value class.
    LHS = DAG.getNode(ISD::EXTRACT_ELEMENT, Dl, MVT::f64, LHS,
                      DAG.getConstant(1, Dl, MVT::i32));
    VT = MVT::f64;
  }
```
I do not think we need add assert here

https://github.com/llvm/llvm-project/pull/204170