[SVE] Eliminate calls to default-false VectorType::get() from AggressiveInstCombine

[ctetreau]

rebase

llvm/lib/Transforms/AggressiveInstCombine/TruncInstCombine.cpp

Type *TruncInstCombine::getBestTruncatedType() {
   return IntegerType::get(CurrentTruncInst->getContext(), MinBitWidth);
 }
 
 /// Given a reduced scalar type \p Ty and a \p V value, return a reduced type
 /// for \p V, according to its type, if it vector type, return the vector
 /// version of \p Ty, otherwise return \p Ty.
 static Type *getReducedType(Value *V, Type *Ty) {
   assert(Ty && !Ty->isVectorTy() && "Expect Scalar Type");
-  if (auto *VTy = dyn_cast<VectorType>(V->getType()))
-    return VectorType::get(Ty, VTy->getNumElements());
+  if (auto *VTy = dyn_cast<VectorType>(V->getType())) {
+    // FIXME: should this handle scalable vectors?
+    return FixedVectorType::get(Ty, VTy->getNumElements());
+  }
   return Ty;
 }
 
 Value *TruncInstCombine::getReducedOperand(Value *V, Type *SclTy) {
   Type *Ty = getReducedType(V, SclTy);
   if (auto *C = dyn_cast<Constant>(V)) {
     C = ConstantExpr::getIntegerCast(C, Ty, false);
     // If we got a constantexpr back, try to simplify it with DL info.