[DAGCombiner] check uses more strictly on select-of-binop fold

There are 2 bugs here:
1. We were not checking uses of operand 2 (the false value of the select).
2. We were not checking for multiple uses of nodes that produce >1 result.

Correcting those is enough to avoid the crash in the reduced test based on:
https://llvm.org/PR51612

The additional use check on operand 0 (the condition value of the select)
should not strictly be necessary because we are only replacing one use
with another (whether it makes performance sense to do the transform with
that pattern is not clear). But as noted in the TODO, changing that
uncovers another bug.

Note: there's at least one more bug here - we aren't propagating EVTs
correctly, but I plan to fix that in another patch.