I have an update today with changed use flags:
[ebuild R ] media-libs/libwebp-0.5.2 USE="(-neon)" CPU_FLAGS_X86="sse2%* -avx2% -sse4_1%"
cpu_flags_x86_sse2 was previously enabled but is getting disabled:
# equery u libwebp
[ Legend : U - final flag setting for installation]
[ : I - package is installed with flag ]
[ Colors : set, unset ]