[InstCombine] (A + 1) + (B ^ -1) --> A - B

Turn canonicalized subtraction back into (-1 - B) and combine it with (A + 1) into (A - B).
This is similar to the folding already done for (B ^ -1) + Const into (-1 + Const) - B.

Differential Revision: https://reviews.llvm.org/D48535

llvm-svn: 335579