postinst runs after prerm when updating

I haven't been able to create an minimal working example yet, but I'm close to it.

It seems to boil down to this:
```
msi {
postinst = '''
set shell = CreateObject("WScript.Shell")
shell.LogEvent 4, "postinst executed"
'''
prerm = '''
set shell = CreateObject("WScript.Shell")
shell.LogEvent 4, "prerm executed"
'''
}
```

The InstallExecuteSequence from the wxs file:
```

NOT Installed OR REINSTALL OR UPGRADINGPRODUCTCODE

REMOVE


```

When you install, you get:

```
postinst executed
```

When you remove an installed version, you get:
```
prerm executed
```

When you **install a new version over an existing one**, you get:
```
prerm executed
postinst executed
postinst executed
```

Would you agree that this is a bug?

Start with the msi configuration and the generated InstallExecuteSequence shown in the report. Reproduce an upgrade over an existing installation and trace the Prerm_Script0 and Postinst_Script0 custom actions; done means prerm runs before postinst and postinst is emitted only once.

Written by the indexing model from the issue text.