AMDGPU: Make various vector undefs legal

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.

(cherry picked from commit 7a846240)