[SelectionDAGBuilder] Hide existence of ConstantDataVector vector from visitGetElementPtr.

ConstantDataVector is a specialized verison of ConstantVector
that stores data in a packed array of bits instead of as
individual pointers to other Constants. But we really shouldn't
expose that if we can void it. And we should handle regular
ConstantVector equally well.

This removes a dyn_cast to ConstantDataVector and just calls
getSplatValue directly on a Constant* if the type is a vector.

llvm-svn: 370018