Replace getAs with castAs, dyn_cast with cast (NFC) (#72600)

Make the code clear that nullptrs are not expected.

Author: mikerice1969

clang/lib/AST/ASTContext.cpp

@@ -4014,8 +4014,8 @@ QualType ASTContext::getVectorType(QualType vecType, unsigned NumElts,
   assert(vecType->isBuiltinType() ||
          (vecType->isBitIntType() &&
           // Only support _BitInt elements with byte-sized power of 2 NumBits.
-          llvm::isPowerOf2_32(vecType->getAs<BitIntType>()->getNumBits()) &&
-          vecType->getAs<BitIntType>()->getNumBits() >= 8));
+          llvm::isPowerOf2_32(vecType->castAs<BitIntType>()->getNumBits()) &&
+          vecType->castAs<BitIntType>()->getNumBits() >= 8));
 
   // Check if we've already instantiated a vector of this type.
   llvm::FoldingSetNodeID ID;

llvm/lib/Frontend/OpenMP/OMPIRBuilder.cpp

@@ -4402,7 +4402,7 @@ static void updateNVPTXMetadata(Function &Kernel, StringRef Name, int32_t Value,
   // Update the "maxntidx" metadata for NVIDIA, or add it.
   MDNode *ExistingOp = getNVPTXMDNode(Kernel, Name);
   if (ExistingOp) {
-    auto *OldVal = dyn_cast<ConstantAsMetadata>(ExistingOp->getOperand(2));
+    auto *OldVal = cast<ConstantAsMetadata>(ExistingOp->getOperand(2));
     int32_t OldLimit = cast<ConstantInt>(OldVal->getValue())->getZExtValue();
     ExistingOp->replaceOperandWith(
         2, ConstantAsMetadata::get(ConstantInt::get(
@@ -4441,7 +4441,7 @@ OpenMPIRBuilder::readThreadBoundsForKernel(const Triple &T, Function &Kernel) {
   }
 
   if (MDNode *ExistingOp = getNVPTXMDNode(Kernel, "maxntidx")) {
-    auto *OldVal = dyn_cast<ConstantAsMetadata>(ExistingOp->getOperand(2));
+    auto *OldVal = cast<ConstantAsMetadata>(ExistingOp->getOperand(2));
     int32_t UB = cast<ConstantInt>(OldVal->getValue())->getZExtValue();
     return {0, ThreadLimit ? std::min(ThreadLimit, UB) : UB};
   }