[SVE] Bail from cstfp_pred_ty before walking scalable vector

[ctetreau]

Summary

Bail out in cstfp_pred_ty before calling getNumElements and walking the
vector if the vector is scalable.

[sdesmalen]

LGTM. Because we can't construct a scalable constant other than a splat, I guess there isn't really a way we can write a test for this change.

llvm/include/llvm/IR/PatternMatch.h

template <typename Predicate> struct cstfp_pred_ty : public Predicate {
   template <typename ITy> bool match(ITy *V) {
     if (const auto *CF = dyn_cast<ConstantFP>(V))
       return this->isValue(CF->getValueAPF());
     if (V->getType()->isVectorTy()) {
       if (const auto *C = dyn_cast<Constant>(V)) {
         if (const auto *CF = dyn_cast_or_null<ConstantFP>(C->getSplatValue()))
           return this->isValue(CF->getValueAPF());
 
+        // Number of elements of a scalable vector unknown at compile time
+        if (isa<ScalableVectorType>(V->getType()))
+          return false;
+
         // Non-splat vector constant: check each element for a match.
         unsigned NumElts = cast<VectorType>(V->getType())->getNumElements();
         assert(NumElts != 0 && "Constant vector with no elements?");
         bool HasNonUndefElements = false;
         for (unsigned i = 0; i != NumElts; ++i) {
           Constant *Elt = C->getAggregateElement(i);
           if (!Elt)
             return false;