[X86] Avoid extra (PMADDUBSW(X,AND(Y)) in <X x i8> multiplication (#168262) On SSSE3 targets we use PMADDUBSW of odd/even with suitable masking to avoid having to extend/truncate with `<X x i16>` types and avoid additional Port0/5 pressure. However, lower i8 elements in the pair can safely use PMULLW directly without any pre-masking as we will only use the lower i8 bits of the result which is only affected by the lower i8 of the inputs.
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