[IR] remove 'NoNan' param when creating FP reductions

This is no-functional-change-intended (AFAIK, we can't
isolate this difference in a regression test).

That's because the callers should be setting the IRBuilder's
FMF field when creating the reduction and/or setting those
flags after creating. It doesn't make sense to override this
one flag alone.

This is part of a multi-step process to clean up the FMF
setting/propagation. See PR35538 for an example.