Non-canonical weak types can lead to failed promotion

[jakevdp]

from jax import lax
x = lax.convert_element_type(1, 'int16', weak_type=True)
print(x)
# DeviceArray(1, dtype=int16)

print(x + 1)
# TypeError: add requires arguments to have the same dtypes, got int16, int32.

The reason this happens is due to something @mattjj noted in #6000; dtypes.result_type essentially canonicalizes all weak types before doing any promotion

[jakevdp]

Might also be related to #6018...