Improve warning for unreachable/overlapping pattern to also take guards into account

Currently, the following program

f :: a -> Bool
f x = case x of
  _ |  False -> False
  _          -> True

generates the warning

warning.curry, line 3.3: Warning:
    Pattern matches are unreachable
    In a case alternative:
      _ -> ...

This is obviously wrong, and is caused by the fact that the guard is not taken into account. A reasonable approach would be to consider only the following guards to always succeed: True, success, otherwise.

In addition, this information may be used to eliminate unreachable alternatives.