[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 #1 from krux <hoganmeier at gmail dot com> ---
Something like -march=armv8-a -mfpu=neon-fp-armv8 does not work either.
https://godbolt.org/z/MpBQ0I