Pattern match warnings are per Match, not per GRHS

Motivation

Consider this binding:

f :: () -> ()
f _
  | False = ()
  | otherwise = ()

This currently won't generate a warning. This program, however, will:

f :: () -> ()
f _
  | False = ()
f _       = ()

This is because we currently report redundancy and inaccesibility warnings per match, not per guarded RHS.

Proposal

Report warnings per guarded RHS.