split 64-mult hashing into two 32-bit multiplies
split 64-mult hashing into two 32-bit multiplies

Speed-wise equivalent on x86 and ARM (maybe a tad faster, hard to tell).

Note that the two 32-bit multiples are not strictly equivalent
to the 64-bit one, since we're missing one carry propagation.
In practice, no observable difference was seen because of this
slightly different hashing result.

Change-Id: I8f2381175eae1cb20dabf149e6b27e1768fba6ab

kalli123/webm.libwebp,Aexyn/libwebp,webmproject/libwebp,imazen/libwebp,kalli123/webm.libwebp,jacklicn/libwebp,Suvarna1488/webm.libwebp,ya7lelkom/libwebp,ya7lelkom/libwebp,ttyangf/libwebp,redengineer/libwebp,zofuthan/libwebp,rxl194/libwebp,ya7lelkom/libwebp,PKRoma/libwebp,ya7lelkom/libwebp,jiehu5114/libwebp,abwiz0086/webm.libwebp,rxl194/libwebp,ttyangf/libwebp,yohunl/libwebp,gshORTON/webm.libwebp,gshORTON/webm.libwebp,gshORTON/webm.libwebp,rxl194/libwebp,Suvarna1488/webm.libwebp,rxl194/libwebp,abwiz0086/webm.libwebp,jiehu5114/libwebp,jacklicn/libwebp,yohunl/libwebp,zofuthan/libwebp,imazen/libwebp,zofuthan/libwebp,kalli123/webm.libwebp,abwiz0086/webm.libwebp,jacklicn/libwebp,imazen/libwebp,Aexyn/libwebp,Aexyn/libwebp,Suvarna1488/webm.libwebp,imazen/libwebp,yohunl/libwebp,redengineer/libwebp,yohunl/libwebp,abwiz0086/webm.libwebp,jiehu5114/libwebp,redengineer/libwebp,rxl194/libwebp,webmproject/libwebp,PKRoma/libwebp,ya7lelkom/libwebp,webmproject/libwebp,webmproject/libwebp,Suvarna1488/webm.libwebp,jacklicn/libwebp,PKRoma/libwebp,gshORTON/webm.libwebp,PKRoma/libwebp,ttyangf/libwebp,webmproject/libwebp,kalli123/webm.libwebp,redengineer/libwebp,ttyangf/libwebp,kalli123/webm.libwebp,PKRoma/libwebp,jiehu5114/libwebp,jiehu5114/libwebp,Suvarna1488/webm.libwebp,abwiz0086/webm.libwebp,gshORTON/webm.libwebp,Aexyn/libwebp,yohunl/libwebp,zofuthan/libwebp,imazen/libwebp,ttyangf/libwebp,PKRoma/libwebp,zofuthan/libwebp,Aexyn/libwebp,jacklicn/libwebp,redengineer/libwebp