[InstCombine] Ensure constant folding in binop of select fold

When folding a binop into a select, we need to ensure that one
of the select arms actually does constant fold, otherwise we'll
create two binop instructions and perform the reverse transform.

Ensure this by performing an explicit constant folding attempt,
and failing the transform if neither side simplifies.

A simple alternative here would have been to limit the fold to
ImmConstants, but given the current representation of scalable
vector splats, this wouldn't be ideal.