Experimentally use amount of intra-group parallelism as threshold.
Experimentally use amount of intra-group parallelism as threshold.

This should make tuning for intra-group parallelism much easier.  On
the other hand, tuning is now a *necessity* for intra-group
parallelism (because the default threshold of 10000 or whatever is far
beyond the max group size).

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