[NFC] [Coroutines] Add a fastpath when computing the cross suspend point information

Mitigate https://github.com/llvm/llvm-project/issues/62348

The root cause for the above issue is that we used a textbook dataflow
analysis for the cross suspend point information. The analysis is
powerful but not scaling.

It is not easy to improve the current algorithm and the patch tries to
prune some branches to mitigate the problems.

Before the patch:

```
n: 20000

real	0m11.081s
user	0m10.597s
sys	0m0.320s

n: 40000

real	0m32.927s
user	0m31.403s
sys	0m1.043s

n: 60000

real	1m2.145s
user	0m58.903s
sys	0m2.268s

n: 80000

real	1m47.143s
user	1m41.630s
sys	0m3.857s

n: 100000

real	2m34.758s
user	2m26.587s
sys	0m5.922s
```

After the patch:

```
n: 20000

real	0m10.418s
user	0m9.945s
sys	0m0.311s

n: 40000

real	0m27.884s
user	0m26.430s
sys	0m1.036s

n: 60000

real	0m52.420s
user	0m49.321s
sys	0m2.267s

n: 80000

real	1m25.389s
user	1m20.247s
sys	0m3.856s

n: 100000

real	2m4.275s
user	1m56.405s
sys	0m5.975s
```

This patch intended to be a NFC patch.