Fix for infix decl w/ infix data constructor.
GHC was rejecting this:

    infix 2 |-
    ps  |-  q:qs = undefined

It parses the def as ((ps |- q) : qs), and doesn't have the fixity info
around at the point where it decides what is being defined.  Lacking
anything else to go on, it decides that `:' is being defined.

Fortunately, we don't really need fixity info to parse this correctly,
as a data constructor is always the wrong choice ;-)  The fix is to
dive into the left-hand-side until we find a non-data constructor.

This is naive - consider the case where `|-' has a high precedence.
Fortunately, someone clever put in a static check later on, presumably
at the point where we have all the fixity info, that rejects the definition
as bogus.  Yeah!