[XLA:CPU] Fuse constants and iota

Fusing constants mostly makes graphs more readable and reduces the number of
fusion arguments. We only do this if the fusion has at least 2 other
instructions so we don't form lots of 2-instruction fusions where one is a
constant. Fusing iota is actually important for performance, we never want to
materialize those.

PiperOrigin-RevId: 236280614