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

The current legality check for folding with EVL has incomplete
verification for VF.
This patch fixes the VF check, ensuring that tail folding with EVL is
enabled only when a scalable VF is available. This allows loops that
prefer tail folding with EVL but cannot use scalable VF vectorization to
still be vectorized using a fixed VF, rather than abandoning
vectorization entirely.

Author: Mel-Chen

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -1454,9 +1454,10 @@ class LoopVectorizationCostModel {
     // FIXME: Investigate opportunity for fixed vector factor.
     // FIXME: support fixed-order recurrences by fixing splice of non VFxUF
     // penultimate EVL.
-    bool EVLIsLegal =
-        UserIC <= 1 && TTI.hasActiveVectorLength(0, nullptr, Align()) &&
-        !EnableVPlanNativePath && Legal->getFixedOrderRecurrences().empty();
+    bool EVLIsLegal = UserIC <= 1 && IsScalableVF &&
+                      TTI.hasActiveVectorLength(0, nullptr, Align()) &&
+                      !EnableVPlanNativePath &&
+                      Legal->getFixedOrderRecurrences().empty();
     if (!EVLIsLegal) {
       // If for some reason EVL mode is unsupported, fallback to
       // DataWithoutLaneMask to try to vectorize the loop with folded tail
@@ -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();
+  setTailFoldingStyles(ContainsScalableVF, UserIC);
   if (foldTailByMasking()) {
     if (getTailFoldingStyle() == TailFoldingStyle::DataWithEVL) {
       LLVM_DEBUG(
@@ -4120,8 +4122,7 @@ LoopVectorizationCostModel::computeMaxVF(ElementCount UserVF, unsigned UserIC) {
       // Tail folded loop using VP intrinsics restricts the VF to be scalable
       // for now.
       // TODO: extend it for fixed vectors, if required.
-      assert(MaxFactors.ScalableVF.isScalable() &&
-             "Expected scalable vector factor.");
+      assert(ContainsScalableVF && "Expected scalable vector factor.");
 
       MaxFactors.FixedVF = ElementCount::getFixed(1);
     }