Or and Xor with extra bitmaps used _or_many/_xor_many, which apply a single
word count to every input. The count came from pointersOf, whose length is
the maximum over the extra slice only and ignores other, so a longer other
lost its tail words. More generally a uniform count cannot combine operands
of different lengths: a longer source is truncated and a shorter one is read
past its end.
Fold each source in with the single-operand SIMD primitive over its own
length, matching the scalar reference in simd_generic.go. Fixes the amd64
accelerated and AVX-512 branches and the arm64 branch.
upstream
dualfroz