Avoid an -Woverlapping-patterns warning on GHC 9.0
Avoid an -Woverlapping-patterns warning on GHC 9.0

GHC 9.0's pattern-match coverage checker is more clever and flags this guard as
redundant:

```
[ 48 of 111] Compiling Cryptol.TypeCheck.SimpleSolver ( src/Cryptol/TypeCheck/SimpleSolver.hs, /tmp/ghc54946_0/ghc_169.o )

src/Cryptol/TypeCheck/SimpleSolver.hs:37:7: warning: [-Woverlapping-patterns]
    Pattern match is redundant
    In an equation for ‘dbg’: dbg msg x | False = ...
   |
37 |     | False     = ppTrace msg x
   |       ^^^^^
```

While true, we want to keep this code in to keep in from bitrotting. I have
tweaked the code slightly so that it is just clever enough to defeat the
pattern-match coverage checker.

GaloisInc/cryptol,GaloisInc/cryptol,GaloisInc/cryptol