[m-rev.] diff: fold checks of mutables into another pass

[Zoltan Somogyi]

This makes the construction of the HLDS a bit simpler
conceptually.

Zoltan.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: Log.mutable_check
Type: application/octet-stream
Size: 505 bytes
Desc: not available
URL: <http://lists.mercurylang.org/archives/reviews/attachments/20151108/9e34373d/attachment.obj>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: DIFF.mutable_check
Type: application/octet-stream
Size: 12079 bytes
Desc: not available
URL: <http://lists.mercurylang.org/archives/reviews/attachments/20151108/9e34373d/attachment-0001.obj>