Address #11245: Ensure the non-matched list is always non-empty
Address #11245: Ensure the non-matched list is always non-empty

When there is an uncovered vector of length 0 (which in turn means that
it represents a guard failure) print "(incomplete guards)" instead of an
empty list of non-covered vectors.

Reviewed By: bgamari

Differential Revision: https://phabricator.haskell.org/D1717

GHC Trac Issues: #11245

(cherry picked from commit bb7f2e33197e667eb694bd1243f125c722a0a868)

GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc