This is currently only used to decide whether it's worth doing an
"are all lanes running" check at the start of functions--for small
functions, it's not worth the overhead.
The cost is estimated relatively early in compilation (e.g. before
we know if an array access is a scatter/gather or not, before
constant folding, etc.), so there are many known shortcomings.
For the case where we have a regular (i.e. non-'cif') 'if' statement,
the generated code just simply checks to see if any program instance
is running before running the corresponding statements. This is a
lighter-weight check than IfStmt::emitMaskMixed() was performing.
Contrary to claims in 0c2048385, that checkin didn't include the changes
to not run if/else blocks if none of the program instances wanted to be
running them. This checkin fixes that and thus actually fixes issue #74.
