[KnownBitsTest] Align with ConstantRange test infrastructure (NFC)

Align the way we perform exhaustive tests for KnownBits with what
we do for ConstantRange. Test each case separately by specifying
a function on KnownBits and one on APInts. Additionally, specify
a callback that determines which cases are supposed to be optimal,
rather than only correct. Unlike the ConstantRange case there is
a well-defined, unique notion of optimality for KnownBits.

If a failure occurs, print out the inputs, computed result and
exact result. Adjust the printing function to produce the output
in a format that is meaningful for KnownBits, i.e. print the
actual known bits, using ? to signify unknowns and ! to signify
conflicts.