[DAGCombiner] fold 'not' with signbit math

This is a follow-up suggested in D48970. 

Alive proofs:
https://rise4fun.com/Alive/sII

We can eliminate an instruction in the usual select-of-constants 
to bit hack transform by adjusting the add/sub with constant.
This is always a win. 

There are more transforms that are likely wins, but they may need 
target hooks in case some targets do not benefit. 

This is another step towards making up for canonicalizing to 
select-of-constants in rL331486.

llvm-svn: 338132