[NFC][InstCombine] Add FIXME's for getLogBase2() / visitUDivOperand()

These are not correctness issues.

In visitUDivOperand(), if the (potential) divisor is undef, then udiv is
already UB, so it is not incorrect to keep undef as shift amount.

But, that is suboptimal.
We could instead simply drop that select, picking the other operand.

Afterwards, getLogBase2() could assert that there is no undef in divisor.