[platform-expression] fix compare()

[maksqwe]

No description provided.

[ras0219-msft]

Thanks for the fix!

What bad behavior caused you to identify this bug? We'd like to add a test case to improve coverage.

[maksqwe]

Just saw this typo in the code.
Test case: when do some search using "vcpkg search ..." this function compare two expressions, for example in ffmpeg vcpkg.json there are platforms which return wrong result from compare().

{
  "name": "ffmpeg",
  "default-features": false,
  "features": [
    "opengl"
  ],
  "platform": "!uwp & !(windows & arm)"
},
{
  "name": "ffmpeg",
  "default-features": false,
  "features": [
    "dav1d"
  ],
  "platform": "!(uwp | arm | x86 | osx)"
},

Complexy of !uwp & !(windows & arm) is 7, when !(uwp | arm | x86 | osx) is 6 and has bigger length.