regalloc: fix lifetime construction for nested loops

Previously, given something like

```
1  a = ...
2  loop { // outer
3    b = do something with a
4    loop { // inner
5      ...
6      if (b < 0)
7        break 'inner;
8      if (b == 0)
9        return;
10     ...
11   }
12 }
```

Regalloc thought outer goes from 2..6, because 6 is the last place where
flow jumps directly back to 2. So `a` would have the lifetime [1,7).
However if neither the break nor return are taken, the inner loop
repeats and then control could flow back to 7 -> 3. But now the physical
location for `a` might have been clobbered between 8..10, which is
wrong.  This fixes that by making sure the outer loop is considered to
span 2..10.

The way I went about it might not be the best way of doing it. I'm not
100% certain that it's fully correct and will always find the correct
loopend, either. It's surprising it took this long to hit this edge
case.

0 files changed, 0 insertions, 0 deletions