Add a new subgroup to -Wtautological-compare, -Wtautological-overlap-compare,
which warns on compound conditionals that always evaluate to the same value.
For instance, (x > 5 && x < 3) will always be false since no value for x can
satisfy both conditions.

This patch also changes the CFG to use these tautological values for better
branch analysis.  The test for -Wunreachable-code shows how this change catches
additional dead code.

Patch by Anders Rönnholm.