[InstCombine] Don't use AddOne/SubOne to see if two APInts are 1 apart. Use...

[InstCombine] Don't use AddOne/SubOne to see if two APInts are 1 apart. Use APInt operations instead. NFCI

AddOne/SubOne create new Constant objects. That seems heavy for
comparing ConstantInts which wrap APInts. Just do the math on
on the APInts and compare them.

llvm-svn: 366648