[XLA] Cache computations when creating reduces in algebraic simplifier or batchnorm expander

Otherwise we create a lot of identical small computations. This shouldn't have
an effect except for cluttering the HLO, but turns out HloCSE doesn't look
inside of the computation of reduces, effectively never eliminating reduces
that were produced via this code path.

While there clean up some YAGNI, this only worked for F32 anyways, so just
hardcode it.

PiperOrigin-RevId: 196689316