[Bug rtl-optimization/58295] [4.8/4.9 regression] Missed zero-extension elimination in the combiner

[ebotcazou at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58295

--- Comment #5 from Eric Botcazou <ebotcazou at gcc dot gnu.org> ---
> Therefore, we can conclude that the original case tried by the combiner is
> the best way to merge/reduce the redundant zero extension insn.

Yes and, although x86 is the dominant architecture, it shouldn't be allowed to
penalize all the others.  I think we should restrict the effect of r191928, in
particular it makes little sense as-is if you also define PROMOTE_MODE as on
most RISC architectures.