[NFC][CLANG] Fix nullptr dereference issue in checkSizelessVectorShift()

[Manna]

Summary

This patch uses castAs instead of getAs which will assert if the type doesn't match in checkSizelessVectorShift(clang::​Sema &, clang::​ActionResult<clang::​Expr *, true> &, clang::​ActionResult<clang::​Expr *, true> &, clang::​SourceLocation, bool).

[Manna]

Thank you @erichkeane for reviews!

clang/lib/Sema/SemaExpr.cpp

if (LHS.isInvalid())
       return QualType();
   }
 
   RHS = S.UsualUnaryConversions(RHS.get());
   if (RHS.isInvalid())
     return QualType();
 
   QualType LHSType = LHS.get()->getType();
-  const BuiltinType *LHSBuiltinTy = LHSType->getAs<BuiltinType>();
+  const BuiltinType *LHSBuiltinTy = LHSType->castAs<BuiltinType>();
   QualType LHSEleType = LHSType->isVLSTBuiltinType()
                             ? LHSBuiltinTy->getSveEltType(S.getASTContext())
                             : LHSType;
 
   // Note that RHS might not be a vector
   QualType RHSType = RHS.get()->getType();
-  const BuiltinType *RHSBuiltinTy = RHSType->getAs<BuiltinType>();
+  const BuiltinType *RHSBuiltinTy = RHSType->castAs<BuiltinType>();
   QualType RHSEleType = RHSType->isVLSTBuiltinType()
                             ? RHSBuiltinTy->getSveEltType(S.getASTContext())
                             : RHSType;
 
   if ((LHSBuiltinTy && LHSBuiltinTy->isSVEBool()) ||
       (RHSBuiltinTy && RHSBuiltinTy->isSVEBool())) {
     S.Diag(Loc, diag::err_typecheck_invalid_operands)
         << LHSType << RHSType << LHS.get()->getSourceRange();