An answer to another question provides a quote from a canon novel which suggests that the Empire is anti-alien:
Palpatine and his propaganda machine worked to drive that nail of
bigotry deeper by demonstrating how the old Jedi thugs and the
scumfroth rebels consisted of many more nonhumans than humans. You
could trust a human, the Empire said; aliens would always betray you.
Star Wars: Aftermath: Life Debt
Furthermore, Wookieepedia's article on xenophobia cites Star Wars Propaganda: A History of Persuasive Art in the Galaxy in stating that the Empire classified several non-human species (such as Wookiees) as "non-sentient". The Empire also enslaved several non-human species, such as the Wookiees and Mon Calamari, though there are non-xenophobic reasons for the Empire to do this (the Wookiees' strength makes them useful for hard labor, and the Mon Calamari proved a particularly rebellious species).
Also, the Rebel Alliance accused the Empire of an anti-alien policy in its Declaration of Rebellion:
You have instituted a policy of blatant racism and genocide against the nonhuman peoples of the Galaxy.
While the Declaration of Rebellion specifically accuses the Empire of instituting an anti-alien policy, the Rebels might have been lying and/or stretching the truth in their declaration.
Finally, it's worth mentioning that humans also seem to be disproportionately represented among Imperial officers (though there are also some high-ranking non-humans, such as Mas Amedda).
That said, I'm unaware of a specific Imperial law or policy which states that humans are superior than non-humans and/or which promotes discrimination against non-humans.