nir: add nir_type_conversion_op()

nir: add nir_type_conversion_op()

This function returns the nir_op corresponding to the conversion between
the given nir_alu_type arguments.

This function lacks support for integer-based types with bit_size != 32
and for float16 conversion ops.

v2:
- Improve readiness of the code and delete cases that don't happen now (Jason)

Signed-off-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>