[LoopDeletion] Exploit undef Phi inputs when symbolically executing 1st iteration

Follow-up on Roman's idea expressed in D103959.
- If a Phi has undefined inputs from live blocks:
   - and no other inputs, assume it is undef itself;
   - and exactly one non-undef input, we can assume that all undefs are equal to this input.

Differential Revision: https://reviews.llvm.org/D104618
Reviewed By: lebedev.ri, nikic