Change dyn_cast<> then assert to cast<> per review suggestions

Author: calebwat

llvm/lib/Transforms/Vectorize/VectorCombine.cpp

@@ -2050,10 +2050,8 @@ bool VectorCombine::foldShuffleOfSelects(Instruction &I) {
        (SI0FOp->getFastMathFlags() != SI1FOp->getFastMathFlags())))
     return false;
 
-  auto *SrcVecTy = dyn_cast<FixedVectorType>(T1->getType());
-  auto *DstVecTy = dyn_cast<FixedVectorType>(I.getType());
-  assert(SrcVecTy && DstVecTy &&
-         "Expected shuffle/select vector types to be defined");
+  auto *SrcVecTy = cast<FixedVectorType>(T1->getType());
+  auto *DstVecTy = cast<FixedVectorType>(I.getType());
 
   auto SK = TargetTransformInfo::SK_PermuteTwoSrc;
   auto SelOp = Instruction::Select;