Ok, break down and add some cast<>'ing helper methods to the Type class

to reduce the number of cast<>'s we have.  This allows someone to use
things like Ty->getVectorNumElements() instead of 
cast<VectorType>(Ty)->getNumElements() when you know that a type is a
vector.

It would be a great general cleanup to move the codebase to use these,
I will do so in the code I'm touching.

llvm-svn: 148999