[Bug target/88013] can't vectorize rgb to grayscale conversion code

[hoganmeier at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88013

--- Comment #4 from krux <hoganmeier at gmail dot com> ---
On x64 indeed both compilers generate a huge amount of code.
https://godbolt.org/z/TH7mqn