While looking into PR 45416, I came across:
/* Check for |= or &= of a bitfield of size one into another bitfield
of size 1. In this case, (unless we need the result of the
assignment) we can do this more efficiently with a
test followed by an assignment, if necessary.
From what I am reading, this code does nothing any more as TREE_CODE (rhs) will never be BIT_IOR_EXPR or BIT_AND_EXPR.