[Bug tree-optimization/104408] SLP discovery fails due to -Ofast rewriting

[tnfchris at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104408

--- Comment #1 from Tamar Christina <tnfchris at gcc dot gnu.org> ---
In particular, the rewrite should probably be gated on the expression being
single use.