[Bug rtl-optimization/92656] The zero_extend insn can't be eliminated in the combine pass

[segher at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92656

--- Comment #4 from Segher Boessenkool <segher at gcc dot gnu.org> ---
The whole reg_stat thing cannot ever reliably track known bits.  We need
some other mechanism to do this, something that *is* reliable, and does
not give different results if you try combinations in a different order.
Something quite like dataflow.  This then could also be used in other
passes, of course.