Multi-measure rests not compressed when \partial is used in middle of score

[Nate Whetsell]

Hi there,

First, thank you for all your work on LilyPond.

Within \compressMMRests, if a multi-measure rest is preceded by a \partial 
that’s not at the beginning of the score, it appears that the multi-measure 
rest isn’t compressed. Here’s a LilyPond example that illustrates the issue:

```
\version "2.20.0"
\compressMMRests {
  c'1
  \partial 4 c'4
  R1*42 % These rests are not compressed.
}
```

All the best,
Nate