[X86][SSE] lowerShuffleAsBitRotate - lower to vXi8 shuffles to ROTL on pre-SSSE3 targets

Without PSHUFB we are better using ROTL (expanding to OR(SHL,SRL)) than using the generic v16i8 shuffle lowering - but if we can widen to v8i16 or more then the existing shuffles are still the better option.