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

These are reassociated versions of the same pattern and
similar transforms as in rL338200 and rL338118.

The motivation is identical to those commits:
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: 338221