[DAGCombiner] sub X, 0/1 --> add X, 0/-1

[DAGCombiner] sub X, 0/1 --> add X, 0/-1

This extends the existing transform for:
add X, 0/1 --> sub X, 0/-1
...to allow the sibling subtraction fold.

This pattern could regress with the proposed change in D57401.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@352680 91177308-0d34-0410-b5e6-96231b3b80d8