NaN fuzzing improvements

[kripken]

• make DE_NAN avoid creating nan literals in the first place
• add a reducer option --denan to not introduce nans in destructive reduction
• add a Literal::isNaN() method
• also remove the default exception logging from the fuzzer js glue, which is a source of non-useful VM differences (like nan nondeterminism)

Background: trying to get fuzzing on jsc working despite this open issue: https://bugs.webkit.org/show_bug.cgi?id=175691

[kripken]

Also added an option --no-fuzz-nans to make it easy to avoid nans when fuzzing (without hacking the source and recompiling).