[TF:XLA] Fix string to HLO opcode conversion for atan2, complex, imag and real.

Make sure that we can't forget opcodes by auto-generating the conversion
functions.

Add auto-generated functions to test HLOs for properties (like IsVariadic,
IsComparison, etc.)

This makes changing HLO more robust and easier because there are fewer places
to update when adding or removing an HLO opcode.

Also:
* Fix IsElementwiseBinary for atan2.
* Add a unit test for HLO opcode helpers.
* Express IsElementwiseBinary in terms of IsElementwise() and operand_count()
  to avoid having to keep the two in sync manually.
PiperOrigin-RevId: 174069664