https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93565
--- Comment #13 from Segher Boessenkool <segher at gcc dot gnu.org> ---
nonzero_bits is not reliable. We also cannot really do what you propose
here, all of this is done for *every* combination.
We currently generate
(set (reg/v:SI 96 [ a ])
(and:SI (reg:SI 104)
(const_int 14 [0xe])))
(set (reg:DI 99 [ a ])
(and:DI (subreg:DI (reg:SI 104) 0)
(const_int 14 [0xe])))
If we can somehow see the first one is just the lowpart subreg of the second,
we can handle it the same as the first case.
