[SCCP] Use constant ranges for select, if cond is overdefined.

For selects with an unknown condition, we can approximate the result by
merging the state of both options. This automatically takes care of
the case where on operand is undef.

Reviewers: davide, efriedma, mssimpso

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D71935