[SCCP] Only handle unknown lattice values in resolvedUndefsIn()

This is a minor refinement of resolvedUndefsIn(), mostly for clarity.
If the value of an instruction is undef, then that's already a legal
final result -- we can safely rauw such an instruction with undef.
We only need to mark unknown values as overdefined, as that's the
result we get for an instruction that has not been processed because
it has an undef operand.

Differential Revision: https://reviews.llvm.org/D128251