https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86882
--- Comment #3 from Segher Boessenkool <segher at gcc dot gnu.org> --- combine creates a clobber of const_int 0 to make sure a pattern will not match. That is valid RTL, but further patterns constructed from that are not (clobber on a rhs). The upper bits of a paradoxical subregs are defined in many cases, see SUBREG_PROMOTED_VAR_P; does that apply here?
