CodeGen: Handle ConstantVector and undef in WinCOFF constant pools

The constant pool entry code for WinCOFF assumed that vector constants
would be formed using ConstantDataVector, it did not expect to see a
ConstantVector.  Furthermore, it did not expect undef as one of the
elements of the vector.

ConstantVectors should be handled like ConstantDataVectors, treat Undef
as zero.