lossless: Speed up ComputeCacheEntropy by 40 %
lossless: Speed up ComputeCacheEntropy by 40 %

a total impact of 1 % on encoding speed

This allows for performance neutral removal of the binary search
in cache bits selection. This will give a small improvement in
compression density.

Change-Id: If5d4d59460fa1924ce71af977320834a47c2054a

zofuthan/libwebp,Aexyn/libwebp,zofuthan/libwebp,rxl194/libwebp,imazen/libwebp,zofuthan/libwebp,PKRoma/libwebp,rxl194/libwebp,rxl194/libwebp,zofuthan/libwebp,webmproject/libwebp,redengineer/libwebp,imazen/libwebp,yohunl/libwebp,imazen/libwebp,zofuthan/libwebp,rxl194/libwebp,webmproject/libwebp,webmproject/libwebp,redengineer/libwebp,yohunl/libwebp,webmproject/libwebp,ya7lelkom/libwebp,yohunl/libwebp,Aexyn/libwebp,ya7lelkom/libwebp,imazen/libwebp,ya7lelkom/libwebp,ya7lelkom/libwebp,yohunl/libwebp,redengineer/libwebp,webmproject/libwebp,ttyangf/libwebp,ttyangf/libwebp,ttyangf/libwebp,redengineer/libwebp,PKRoma/libwebp,imazen/libwebp,ttyangf/libwebp,Aexyn/libwebp,PKRoma/libwebp,ya7lelkom/libwebp,PKRoma/libwebp,PKRoma/libwebp,Aexyn/libwebp,ttyangf/libwebp,Aexyn/libwebp,PKRoma/libwebp,yohunl/libwebp,redengineer/libwebp,rxl194/libwebp