Allow return in tailrec position

[mbovel]

Implement lampepfl/dotty-feature-requests#261.

Joint work with @ghostbuster91 in the framework of the issue spree.

[sjrd]

This shouldn't create any issues now.

However, to be complete, you'll have to actually recurse in all trees all the time. See this comment:
https://github.com/lampepfl/dotty/blob/a34977551e9e13090e6b5201f7d91879464f991c/compiler/src/dotty/tools/dotc/transform/TailRec.scala#L29-L31
Now any nested tree could contain a recursive call, which was not the case before.

Also, this will need more tests before it's ready to merge. ;)

[sjrd]

Looks good to me.

There might be concerns about the performance impact, since now checking needs to be done everywhere all the time. So perhaps ask the performance bot?

[mbovel]

test performance please

[dottybot]

performance test scheduled: 1 job(s) in queue, 0 running.

[dottybot]

Performance test finished successfully:

Visit https://dotty-bench.epfl.ch/14067/ to see the changes.

Benchmarks is based on merging with master (eb75a1a)

[mbovel]

@sjrd could we merge this or do we need more changes or measurements?

[sjrd]

The benchmark results look good to me. It's good to go as far as I'm concerned.