[NFC] Make Type::isVectorTy call isa<VectorType>

[efriedma]

This will fail to compile if someone includes Type.h without including DerivedTypes.h.

[ctetreau]

address code reveiw issues

[ctetreau]

In D77425#1960531, @efriedma wrote:

This will fail to compile if someone includes Type.h without including DerivedTypes.h.

Right you are. I guess I didn't notice because in practice everybody includes DerivedType. I changed it to work like the old vector getters.

I actually wanted to get rid of this function too, but there are literally thousands of usages of it...

[efriedma]

LGTM

[ctetreau]

rebase such that this depends on no other changes

[dblaikie]

What's the motivation for this change compared to the previous code? This looks problematic in ways I can't really see a fix for. While the originally proposed patch would fail to compile if you included Type.h without DerivedType.h - the new version will compile but fail to link (a & probably should fail to compile - it will if you compile with -Wundefined-inline)

llvm/include/llvm/IR/Type.h

 namespace llvm {
 
 template<class GraphType> struct GraphTraits;
 class IntegerType;
 class LLVMContext;
 class PointerType;
 class raw_ostream;
 class StringRef;
+class VectorType;
 
 /// The instances of the Type class are immutable: once they are created,
 /// they are never changed.  Also note that only one instance of a particular
 /// type is ever created.  Thus seeing if two types are equal is a matter of
 /// doing a trivial pointer comparison. To enforce that no two equal instances
 /// are created, Type instances can only be created via static factory methods
 /// in class Type and in derived classes.  Once allocated, Types are never
 /// free'd.
public:
 
   /// True if this is an instance of PointerType.
   bool isPointerTy() const { return getTypeID() == PointerTyID; }
 
   /// Return true if this is a pointer type or a vector of pointer types.
   bool isPtrOrPtrVectorTy() const { return getScalarType()->isPointerTy(); }
 
   /// True if this is an instance of VectorType.
-  bool isVectorTy() const { return getTypeID() == VectorTyID; }
+  bool isVectorTy() const { return isa<VectorType>(this); }
 
   /// Return true if this type could be converted with a lossless BitCast to
   /// type 'Ty'. For example, i8* to i32*. BitCasts are valid for types of the
   /// same size only where no re-interpretation of the bits is done.
   /// Determine if this type could be losslessly bitcast to Ty
   bool canLosslesslyBitCastTo(Type *Ty) const;
 
   /// Return true if this type is empty, that is, it has no elements or all of