Surprisingly these were getting legalized to something
zero initialized.
This fixes an infinite loop when combining some vector types.
Also fixes zero initializing some undef values.
SimplifyDemandedVectorElts / SimplifyDemandedBits are not checking
for the legality of the output undefs they are replacing unused
operations with. This resulted in turning vectors into undefs
that were later re-legalized back into zero vectors.
