GPUTracking: replace the merger's goto with helper lambdas

[ktf]

MSL supports neither goto nor labels. The label sat at the end of the loop body,
so each jump was a continue for the outer loop that could not be written as one
because it was issued from an inner loop.

Two lambdas take over the chain walks that did the jumping: one advances a track
along its segment chain and reports whether the target was met on the way, the
other reports whether the target sits on either neighbour chain. Each jump then
becomes a plain continue at the top level of the loop body. A null target lets
the first one also cover the four walks that never tested for one, leaving the
file three lines shorter than it is with the goto.

Checked against the goto version by transcribing both loop bodies into a harness
and comparing the skip decision and the resulting segment and neighbour links of
every track over 400000 random link graphs, 71% of which took a skip path and
100000 of which reached the merging stage: no differences.

[davidrohr]

Hm, I don't like that we need more instructions, but I do not think that there is a better solution. I do not believe this is performance critical, but I want to check at least that it does not change the performance. An alternative to make the code cleaner could be to use a lambda or a separate function.

[davidrohr]

I will try to run with this and O2/dev today and compare the performance on GPU, but I do not think there will be a difference. If we could have a version with a lambda or a function and return instead of goto, I'd prefer that for the syntax, and I assume also that would not change the performance.

[ktf]

Version with the lambda now pushed in place of the old commit. It's even 3 lines shorter... ;-)