[LegalizeDAG] Don't bother with final MUL+SRL stage for byte CTPOP.

The final stage of CTPOP expansion (v = (v * 0x01010101...) >> (Len - 8)) is completely pointless for the byte (Len = 8) case as it reduces to (v = (v * 0x01...) >> 0), but annoyingly this doesn't always get optimized away. 

Found while investigating generic vector CTPOP expansion (PR32655).

llvm-svn: 344477