Register self loops in the loop forest

[Acture]

Summary

• Register every loop header as a key in forest. A header only got one as a side effect of some body block naming it as parent, so a loop whose body is just the header itself was missing from the forest.
• Function::unroll descends into a child only when forest.count(child) holds, so a nested self loop was never unrolled and kept its backedge. State::addJump redirects that backedge to #sink, dropping the executions that run the inner loop more than once from the refinement query.
• Add a regression test where src returns %n and tgt returns 3 * %n. alive-tv reports it correct without this patch.

Fixes #1305.

Testing

• ninja check

Result: 1423 passed, 1 expected failure.

I could not build master locally as it needs a newer LLVM than I have, so this was run on 0dc2be5 with the same patch applied. The loop forest construction this changes is identical between that commit and master.