Legalize `atan2` analogously to XLA. `atan2` is first reduced to `atan` on the interval [-1, 1] and subsequently approximated. This CL also adds e2e tests for trigonometric approximations. PiperOrigin-RevId: 334794336
To add more approximation lowerings in the future, generalize the pass name. PiperOrigin-RevId: 333340075