https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108477
--- Comment #1 from Uroš Bizjak <ubizjak at gmail dot com> ---
This conversion happens due to th following code in match.pd:
/* If we are XORing or adding two BIT_AND_EXPR's, both of which are and'ing
with a constant, and the two constants have no bits in common,
we should treat this as a BIT_IOR_EXPR since this may produce more
simplifications. */
(for op (bit_xor plus)
(simplify
(op (convert1? (bit_and@4 @0 INTEGER_CST@1))
(convert2? (bit_and@5 @2 INTEGER_CST@3)))
(if (tree_nop_conversion_p (type, TREE_TYPE (@0))
&& tree_nop_conversion_p (type, TREE_TYPE (@2))
&& (wi::to_wide (@1) & wi::to_wide (@3)) == 0)
(bit_ior (convert @4) (convert @5)))))
