[SVE] Fix invalid use of VectorType::getNumElements() in PatternMatch

[ctetreau]

Summary

Update cst_pred_ty to only work on FixedVectorType. It operates on
integers and integer vectors, and returns true if the predicate returns
true for all elements of the vector. This operation is not possible on
scalable vectors. Make this behavior explicit in the code and document
the fact that it only tests fixed width vectors.

Identified by test LLVM.Transforms/InstCombine::nsw.ll

llvm/include/llvm/IR/PatternMatch.h

template <int64_t Val> struct constantint_match {
   }
 };
 
 /// Match a ConstantInt with a specific value.
 template <int64_t Val> inline constantint_match<Val> m_ConstantInt() {
   return constantint_match<Val>();
 }
 
-/// This helper class is used to match scalar and vector integer constants that
-/// satisfy a specified predicate.
+/// This helper class is used to match scalar and fixed width vector integer
+/// constants that satisfy a specified predicate.
 /// For vector constants, undefined elements are ignored.
 template <typename Predicate> struct cst_pred_ty : public Predicate {
   template <typename ITy> bool match(ITy *V) {
     if (const auto *CI = dyn_cast<ConstantInt>(V))
       return this->isValue(CI->getValue());
-    if (V->getType()->isVectorTy()) {
+    if (const auto *FVTy = dyn_cast<FixedVectorType>(V->getType())) {
       if (const auto *C = dyn_cast<Constant>(V)) {
         if (const auto *CI = dyn_cast_or_null<ConstantInt>(C->getSplatValue()))
           return this->isValue(CI->getValue());
 
         // Non-splat vector constant: check each element for a match.
-        unsigned NumElts = cast<VectorType>(V->getType())->getNumElements();
+        unsigned NumElts = FVTy->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;
           if (isa<UndefValue>(Elt))
             continue;

 /// For vectors, this includes constants with undefined elements.
 inline cst_pred_ty<is_zero_int> m_ZeroInt() {
   return cst_pred_ty<is_zero_int>();
 }
 
 struct is_zero {
   template <typename ITy> bool match(ITy *V) {
     auto *C = dyn_cast<Constant>(V);
+    // FIXME: this should be able to do something for scalable vectors
     return C && (C->isNullValue() || cst_pred_ty<is_zero_int>().match(C));
   }
 };
 /// Match any null constant or a vector with all elements equal to 0.
 /// For vectors, this includes constants with undefined elements.
 inline is_zero m_Zero() {
   return is_zero();
 }