[SelectionDAG][GlobalISel] Don't use UnsignedDivisionByConstantInfo for divisor of 1.

The magic algorithm sets IsAdd indication for division by 1 that
the caller had to ignore.

I considered folding the ignore into UnsignedDivisionByConstantInfo,
but we only allow 1 for vectors of mixed visiors. And really what we
want to end up with is undef. Currently, we get to undef via
DemandedElts optimizations using the select instruction. We could
directly emit undef.

Differential Revision: https://reviews.llvm.org/D140940