[XLA] Add tests for edge cases of sqrt(x) and pow(x, 0.5).

These are subtly different:

 - sqrt(-inf) = nan, but
   pow(-inf, 0.5) = +inf

Add exhaustive tests for sqrt, pow(0.5), and move the exhaustive fp16 test for
log into the new file.  Doing this requires a nontrivial rewrite of the
exhaustive test; hopefully this is the last time.

While we're here, we enable a few more exhaustive tests.

Also remove the special-case handling in GPU elemental IR emitter for
pow(x, 0.5) now that we have a sqrt HLO.

PiperOrigin-RevId: 235245101