[llvm] [LV][EVL] Fix the check for legality of folding with EVL. (PR #125678)

[Mel Chen via llvm-commits]

================
@@ -4109,7 +4110,8 @@ LoopVectorizationCostModel::computeMaxVF(ElementCount UserVF, unsigned UserIC) {
   // found modulo the vectorization factor is not zero, try to fold the tail
   // by masking.
   // FIXME: look for a smaller MaxVF that does divide TC rather than masking.
-  setTailFoldingStyles(MaxFactors.ScalableVF.isScalable(), UserIC);
+  bool ContainsScalableVF = MaxFactors.ScalableVF.isNonZero();
----------------
Mel-Chen wrote:

I am unable to make the change you suggested for the following reasons:
`MaxFactors.ScalableVF` is of type `class ElementCount`, and its inherited `operator bool()` is declared as explicit:
```
explicit operator bool() const { return isNonZero(); }
```
This means it cannot be implicitly casted to a boolean except an if statement. Or do you want to use `static_cast<bool>` here?
Additionally, as @lukel97  mentioned, `MaxFactors.ScalableVF.isScalable()` is always true. 
```
  FixedScalableVFPair()
      : FixedVF(ElementCount::getFixed(0)),
        ScalableVF(ElementCount::getScalable(0)) {}
  FixedScalableVFPair(const ElementCount &Max) : FixedScalableVFPair() {
    *(Max.isScalable() ? &ScalableVF : &FixedVF) = Max;
  }
  FixedScalableVFPair(const ElementCount &FixedVF,
                      const ElementCount &ScalableVF)
      : FixedVF(FixedVF), ScalableVF(ScalableVF) {
    assert(!FixedVF.isScalable() && ScalableVF.isScalable() &&
           "Invalid scalable properties");
```
I believe we don’t need to check this.

https://github.com/llvm/llvm-project/pull/125678