[NFC] Use cast instead of dyn_cast for Src and Dst vec types in VecCombine folding (#134432)

SrcVecTy and DstVecTy are used without a null check, and originate from
a dyn_cast. This patch adjusts this to use a fixed cast, since it is not
checked for null before use otherwise, but is semantically guaranteed
from previous checks.

Author: calebwat

llvm/lib/Transforms/Vectorize/VectorCombine.cpp

@@ -2051,8 +2051,8 @@ 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());
+  auto *SrcVecTy = cast<FixedVectorType>(T1->getType());
+  auto *DstVecTy = cast<FixedVectorType>(I.getType());
   auto SK = TargetTransformInfo::SK_PermuteTwoSrc;
   auto SelOp = Instruction::Select;
   InstructionCost OldCost = TTI.getCmpSelInstrCost(