[InstCombine] try to fold 'sub' to 'not'

https://rise4fun.com/Alive/jDd

Patterns with add/sub combos can be improved using
'not' ops. This is better for analysis and may lead
to follow-on transforms because 'xor' and 'add' are 
commutative/associative. It can also help codegen.  

llvm-svn: 338200