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

[npickito at gmail dot com]

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

--- Comment #12 from Kito Cheng <npickito at gmail dot com> ---
> The question sounds self-contradictory...  Anyway, rather than inventing a 
> new 
> hook for each problem, let's try to formulate it in terms of existing hooks.

TARGET_LEGITIMATE_COMBINED_INSN hook is add in r190846
and x86 used it in r190847.