Recommit r333226 "[ValueTracking] Teach computeKnownBits that the result of an...

Recommit r333226 "[ValueTracking] Teach computeKnownBits that the result of an absolute value pattern that uses nsw flag is always positive."

Libfuzzer tests have been fixed to prevent being optimized.

Original commit message:

If the nsw flag is used in the absolute value then it is undefined for INT_MIN. For all other value it will produce a positive number. So we can assume the result is positive.

This breaks some InstCombine abs/nabs combining tests because we simplify the second compare from known bits rather than as the whole pattern. Looks like we can probably fix it by adding a neg+abs/nabs combine to just swap the select operands. N

Differential Revision: https://reviews.llvm.org/D47041

llvm-svn: 333300