Add a ConstantDataVector::getSplatValue() method, for parity with

ConstantVector.  Fix some outright bugs in the implementation of
ConstantArray and Constant struct, which would cause us to not make
one big UndefValue when asking for an array/struct with all undef
elements.  Enhance Constant::isAllOnesValue to work with
ConstantDataVector.

llvm-svn: 149021