Fix async/await decompilation for Roslyn 3.6

[JarLob]

Closes https://github.com/0xd4d/dnSpy/issues/1488

Minimal changes to fix the issue, although it seems async/await decompilation logic in ILSpy 6.x is more complex.

[JarLob]

I just secretly hope @0xd4d doesn't accept the pull request because he/she has almost finished migration to the newest ILSpy version :)

[0xd4d]

Thanks for the PR and for pinging me. I'll have a look!

[0xd4d]

I tried the code, and it works when it's compiled in Debug mode, but when I compile the example in https://github.com/0xd4d/dnSpy/issues/1488 in Release mode, it fails to decompile it because there are only 4 ILAst instructions.

Changing the 5 check in MatchTaskCreationPattern() to 4 instead seems to solve it.

[JarLob]

Thanks, will look into it.

[0xd4d]

I just noticed that there's a body[pos - 3] that could fail since pos = len-2. We'd need to verify that pos-3 >= 0 first. (line 104)