blockedGenReduce: compute sizes in 64 bits.
blockedGenReduce: compute sizes in 64 bits.

This is related to #134.  I think we will just gradually move things
to 64-bit over time.

diku-dk/futhark,diku-dk/futhark,diku-dk/futhark,HIPERFIT/futhark,HIPERFIT/futhark,HIPERFIT/futhark,diku-dk/futhark,diku-dk/futhark